Microfluidic system with a plurality of sequential T-junctions for performing reactions in microdroplets

ABSTRACT

A microfluidic system with a plurality of sequential T-junctions for performing reactions in a plurality of droplets (plugs) of micro- to femtoliter volumes is disclosed. The microfluidic system is configured such that the plurality of plugs are flowing through a loading component, with the loading component splitting the plurality of plugs at each of the plurality of downstream T-junctions and each inlet of the plurality of detachable holding components is configured to be operably coupled with at least one of outlets of the plurality of downstream T-junctions so that the holding components received a plurality of split plugs in the immiscible carrier fluid from the loading component.

RELATED APPLICATIONS

The present application is a continuation under 35 U.S.C. §120 of U.S.Ser. No. 11/082,187, filed Mar. 16, 2005, now U.S. Pat. No. 7,655,480,incorporated herein by reference. The present application is alsorelated to and claims priority pursuant to 35 U.S.C. §119 to U.S. Ser.Nos. 60/623,261 (filed Oct. 29, 2004) and 60/585,801 (filed Jul. 2,2004); both of which are incorporated herein by reference in theirentirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This work was supported by the NIH (R01 EB001903). Additionally, some ofthis work was performed at the MRSEC microfluidic facility funded by theNSF. The government may have certain rights in this application.

TECHNICAL FIELD

The present invention relates to the field of microfluidics. Thisinvention provides microfluidic technology enabling rapid and economicalmanipulation of reactions on the sub-femtoliter to milliliter scale.

BACKGROUND OF THE INVENTION

The ability to run a large number of reactions using minimal reagent isdesirable. Various solutions to this goal have been proposed, includingrobotics, microfluidics, combinatorial chemistry in 96-well plates, etc.The application of these methods to a variety of reactions has beenproposed.

For example, membrane proteins play a crucial role in many cellular andphysiological processes critical to human health. Determination of thestructure of membrane proteins is a critical step in the understandingof their function. X-ray crystallography is a powerful tool for thedetermination of the structure of membrane proteins. Crystallizationconditions for membrane proteins are determined by a large number ofscreening experiments. Membrane proteins are often difficult to produce,therefore miniaturization of the crystallization screens is essentialfor accelerating the structural studies. Crystals of membrane proteinsare often fragile and can be damaged by handling, therefore technologiesare needed to allow for direct in situ testing of the diffractionquality of crystals. Handling nanoliter volumes of solutions of membraneproteins is challenging due to their high viscosity and low surfacetension.

WO 04/104228 describes the use of microfluidic structures for highthroughput screening of protein crystallization. In one embodiment, anintegrated combinatorially mixing chip allows for precise metering ofreagents to rapidly create a large number of potential crystallizationconditions, with possible crystal formation observed on chip. In analternative embodiment, the microfluidic structures may be utilized toexplore phase space conditions of a particular protein/crystallizingagent combination, thereby identifying promising conditions and allowingfor subsequent focused attempts to obtain crystal growth. Unfortunately,this system is expensive to operate; the combinatorially mixing chipsare significantly more expensive than traditional crystallographyplates; the combinatorially mixing chips are water and gas permeable;and are incompatible with organic solvents. Thus, a need exists forrapid, economical crystallization of membrane proteins.

BRIEF SUMMARY OF THE INVENTION

This invention provides a method of conducting a large number ofreactions in parallel, such as crystallizations or assays, allowingrapid and economical reactions.

This invention is particularly well suited for crystallization ofproteins, biomolecules, complexes of biomolecules with bound ligand,etc. This invention allows direct testing of the diffraction quality ofcrystals. The present invention can also be applied in the field ofcombinatorial chemistry, allowing the monitoring of both kinetics andreaction products.

The present invention has one or more of the following attractivefeatures: (1) The method is scalable—increasing the number of reagentsused in a screen does not require more complex fabrication, just alonger receiving component. (2) The method in some embodiments mayinclude the use of spacers in three-phase flow. For example, the use offluorinated carrier fluid can provide protection of plugs and control ofthe surface chemistry, and the use of gas bubbles can prevent aqueousplugs from merging. (3) Arrays may be pre-fabricated by a range ofmethods, from simple methods using syringes, to robotics. Pre-fabricatedarrays of plugs in capillaries may be stored for months, and can be madesterile or prepared under inert atmosphere, expanding the range ofpotential applications. (4) The method is very simple for the enduser—no sophisticated equipment is required on the user's end except asource of constant flow to drive multiple streams to combine. Overall,this method is attractive for applications in which reagents must bestored and used in a simple, reliable format, such as in diagnostics anddetection. In addition, this method may find a wide range of applicationin chemistry and biochemistry, enhancing and miniaturizing currentmethods in which reagents in 96-, 384- and 1536-well plates are storedor distributed, such as combinatorial chemistry, proteincrystallization, and biochemical assays.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1. A schematic illustration of a design of a reusable microfluidicsystem. The plugs 9 in a carrier 6 can be transported either (a) from amicrochannel in a microfluidic device to a capillary 3 or (b) from acapillary to a microchannel on a microfluidic device via an adapter 2.

FIG. 2. Mass-producing holding components containing plugs. a) DeviceSchematic. A large channel containing a series of different plugsseparated by carrier is fed into a splitting component. Each junctionsplits each plug into two separate plugs. After three iterations, 8small holding components are each filled with an identical series ofplugs. b) 3-dimensional view of splitting channels. Since all iterationsrest on a common plane, successive layers can be built up usingmultilayer photolithography. c) a simple T-junction. d) Narrowedjunctions. e) Junctions with multiple outlets.

FIG. 3. Merging with split multiple microchannels. a) Three-dimensionalsection of a microchannel showing a plug merging with a stream flowedthrough a split channel. b) Microphotograph of multiple aqueous streamsoriginating from a single delivery channel merging with plugs of redinorganic dye. c) Merging of a stream (illustrated by a streamcontaining protein) with an array of plugs.

FIG. 4. A schematic illustration of the merging of the array of plugs 7with another array of protein plugs 13. While an application of thisidea to protein crystallization is described below, the same principlesmay be applied to reactions, assays, and analyses. For example, a set ofdiverse reactants may arranged in an array of plugs, and then mergedwith a stream containing the sample.

FIG. 5. Schematic diagrams of methods to bring two plugs into contact.a) Plugs in a channel where wetting of the channel by the plug fluid isprevented. b) Plugs in a channel where wetting of the channel by theplug fluid is allowed. c) Plugs in a channel while flow is establishedand wetting of the channel by the plug fluid is prevented. d) Plugs thatare not flowing in a channel and wetting of the channel by the plugfluid is allowed. e) Plugs in a channel with a photoswitchable monolayerthat initially prevents wetting. f) Plugs in a channel after thephotoswitchable monolayer has been activated with a particularwavelength of light. g) Aqueous plugs inside a holding component. h) Aforce is applied such that the symmetry is broken and a liquid bridgebetween the two plugs is formed. The force may be magnetic, electric,optical, or an accelerating force (such as in centrifugation), etc. Theforce may be exerted on the carrier fluid, the crystallizationsolutions, or both, depending on the configuration of the experiment.Preferably the force is applied normal to the section of the holdingcomponent. If magnetic forces are used, preferably the carrier fluidsand the plug fluids have different magnetic properties. Ifcentrifugation is used, preferably the carrier fluids and the plugfluids have different densities, as is the case when aqueous plug fluidsand fluorinated carrier fluids are used.

FIG. 6. a) An array of plugs of four different reagents in a capillary.The plugs contain KMnO₄, NaCl, CUSO₄ and Fe(SCN)₃, from left to right.The carrier is a fluorocarbon with surfactant. b, c) An array of plugsof different reagents formed in fluorocarbon and separated by airbubbles (dark) in a capillary. In (b) the aqueous plugs are separatedfrom air bubbles by a spacer of fluorocarbon, preventingcross-communication between the plugs. The scale bars are 200 μm.

FIG. 7. A schematic microfluidic setup for controlling distances betweenplugs, and in some cases for merging plugs. The carrier fluid is drainedthrough small side branch channels. (a) The spacing between the plugsdecreases accordingly. (b) The carrier fluid is completely removed andall the contents of the plugs merged into a stream of plug fluid.

FIG. 8. a) PDMS-glass composite system for protein crystallization. b)Synchrotron diffraction image (right) of a thaumatin crystal (left)grown and diffracted inside a plug in a capillary to yield a structurewith better than 1.8 Å resolution (I/σ(I)=4.7 at 1.8 Å).

FIG. 9. Crystallization of the membrane protein FAAH. Plug-basedmicrofluidics can be used to crystallize membrane proteins and performmany crystallization trials with small amounts of sample. a) Amicrophotograph of one meter of Teflon capillary wound around a dime.The capillary contains ˜1,000 nanoliter-sized plugs. Only 10 μL ofmembrane protein solution is needed to set up 1000 crystallizationtrials. b) A magnified portion of the microphotograph in a) showingplugs in the capillary. c) Two microphotographs of membrane protein(FAAH) crystals grown in plugs.

FIG. 10. A flowchart showing one embodiment of the microfluidic systemof the present invention. The loading component prepares a linear arrayof plugs containing a first set of reagents. The loading component isattachable to a holding component such that the array can be transferredwith fluid flow. The holding component can be attached to the combiningcomponent, such that a second reagent or set of reagents can beintroduced into plugs within the array. The array can be moved to areceiving component for further manipulations.

FIG. 11. A schematic illustration of the assay of multiple plugscontaining different enzymes against a single protein substrate 8. Eachplug containing an enzyme 22 is separated from another plug with adifferent enzyme by carrier, a spacer 21, and at least one washing plug23.

FIG. 12. A schematic illustration of a possible geometry for merging aplug 9 with a stream 8 to created a merged plug 10 withoutcross-contamination. A short narrow side channel may be used for thecarrier 6 to aid in snapping off a volume of the merging stream 8. Thevolume of the trapezoidal expansion may be used to define the volume ofthe fluid merged. Other shapes than the trapezoidal expansion may beused to optimize the performance. The main channel shown in bold linesmay be made greater in the z-dimension than the other channels shownwith thinner lines.

FIG. 13. A simple T-junction in PDMS is fitted with a Teflon capillary.The PDMS chip is fabricated as usual by sealing a piece of PDMS withmicrofluidic channels to another flat piece of PDMS. Prior to sealing, aTeflon capillary is placed in the channel. The space between the Tefloncapillary and square microfluidic channel is then filled with uncuredPDMS mixture and cured. The resulting junction can be used to merge anarray of preformed plugs with another stream, or to form plugs. Sincefluorinated carrier fluids preferentially wet a Teflon surface, theTeflon capillaries do not need surface treatment before use. a) shows anarray of preformed plugs (channel b) merging with another stream(channel a). b) shows that to form plugs, an aqueous stream can bepumped in through channel a while the carrier fluid is pumped throughchannel b. At the junction, aqueous droplets, or plugs, formspontaneously and are transported through the Teflon capillary. This isparticularly useful for aqueous detergent solutions, which would nototherwise may be able to form plugs.

FIG. 14. a) Schematic and b) microphotograph of Teflon capillary-PDMScomposite device used for a membrane protein crystallization experiment.Plugs containing membrane protein and precipitants were formed in astream of immiscible carrier fluid composed of perfluorotrialkylamines.The membrane protein used was the apo form of fatty acid amidehydrolase, the carrier fluid was perfluoro-compound FC-70, and theprecipitant was 24% PEG 4000, 0.20 M LiSO4, 0.40 M NaCl, 0.10 M TrispH=8.2. Flow rates were as follows: carrier fluid—1 μL/min; protein—0.5μL/min; each precipitant stream—0.25 μL/min.

FIG. 15. Tubing or capillaries can also be used to connect several chips24. For example, plugs 9 may be formed or merged with a stream 8 on onedevice, and transferred through a tubing and/or a capillary to anotherchip 24. The use of an inserted capillary 3 to make a connection to onedevice, and the use of a funnel-shaped adapter 2 to make a connection toanother device is shown in the figure. A range of connecting methods maybe used in any combination.

FIG. 16. The connection between tubing 25 and a chip 24 may be out ofplane of the chip. Tubing can be connected to the chip through a hole onthe microfluidic chip. Preformed arrays of plugs can be transported tothe microfluidic chip for further manipulation.

FIG. 17. (a) Two arrays of plugs 9 can be stored in the holdingcomponent. The holding component may be configured so that the twoarrays of plugs can be merged, as shown in (b). The merged plugs 10 canbe then transferred to a combining component for further reactions.

FIG. 18. (a) An array of plugs 9 and a stream 8 may also be stored inthe holding component. Merged plugs 10 can be formed prior to transferto a combining component for further reactions, as shown in (b).

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a system for plug-based microfluidicmanipulation of small volumes of solutions. Certain embodiments of thesystem comprise a loading component, a holding component, a combiningcomponent and a receiving component. FIG. 10 illustrates one embodimentof the system of the present invention. The various components can beintegrated into a single system or one or more of the components can bedetachable from the microfluidic system. For example, each componentcould be separately detachable—such that the holding component can be astand alone component that is attachable to both the loading componentand the combining component. In another example, the receiving componentmay be integrated with the combining component

Throughout this disclosure the use of the present invention isexemplified by application to protein crystallography. It should beunderstood that this invention is equally applicable to other reactions.

Plug-based microfluidic manipulation of solutions has been describedpreviously by the present inventors in U.S. Ser. Nos. 10/434,970 and10/765,718, incorporated herein in their entirety by reference.

Loading Component

The loading component comprises at least one microchannel, which issuitable for forming an array comprising fluid plugs that are separatedfrom one another by a carrier and at least one outlet configured to beable to make a fluid connection with a holding component.

The loading component can be any device capable of filling a holdingcomponent with the desired array of plugs separated by the desiredcarrier fluid or fluids. The process could be carried out manually, forinstance with manually operated pipettes or syringes, or through the useof pipetting robots. Alternatively, the loading component can be amicrofluidic device with at least one channel and at least one outletconfigured to be able to make a fluid connection with a receivingcomponent. The loading component could also be based on the type ofdevices described by Laura Lane, The Scientist, Volume 18, Issue 1(34),Jan. 19, 2004, that are capable of preparing small volumes of varyingsolutions. Such devices include, for example, the Agincourt™ systemmanufactured by Syrrx, Gilson's 925 PC Workstation, deCODE'sMatrixMakerm, Matrix Technologies' Hydra® Plus-One System, and theFluidigm Topaz™ Crystallizer. The loading component may or may not havean integral means for sealing or capping the ends of the holdingcomponent.

Holding Component

The holding component is useful for storing and transporting at leastone linear array of plugs.

The detachable holding component has at least one microchannelconfigured so that it can make a fluid connection with the loadingcomponent. When the holding component is detached from the loadingcomponent, the fluid plugs typically do not immediately merge or mix.The detachable holding component may be a preformed cartridge that maybe sold as an independent item of commerce. The terms “holdingcomponent” and “preformed cartridge” are used interchangeably in thisapplication. Examples of holding component materials include, but arenot limited to, PDMS, glass tubes (such as a capillary), tubing (such asTeflon tubing or polyimide tubing), and composite tubing (for example, aglass capillary coated with polyimide, or Teflon tubing inside a glasscapillary). The microchannel of the holding component can have an innerdiameter of similar or differing size to the microchannel of the loadingcomponent, the combining component or the receiving component. Theholding component may be wound, such as in a spiral or a cylinder, andtransported with or without a spool, to facilitate handling.Microchannels may be wound, such as in a spiral or a cylinder. For thepurposes of this application, the plugs in such configurations are stillconsidered to be arranged in a “linear array”.

One embodiment of the present invention is a kit containing a holdingcomponent with at least one linear array of plugs. The holding componentin the kit may have more than one array of plugs. For example, twoarrays of plugs can be stored in the holding component. The holdingcomponent may be configured so that the two arrays of plugs can bemerged. The merged plugs can be then transferred to the combiningcomponent for further reactions. Also, an array of plugs may be storedin the holding component together with a stream of a reagent or solvent.The holding component may be configured to induce merging of the plugswith the stream, and this merged array of plugs may be transferred intothe combining component for further reactions. Such a system will beuseful, for example, for reactions that involve the generation ofunstable species upon mixing of two or more components that are morestable.

In one embodiment, the holding component may be capped or sealed on oneor both ends to facilitate storage or transport. If the carrier fluid isnot volatile, it may not be necessary to seal or cap the ends. Also,unsealed holding components may transported by submerging one or bothends in carrier fluid.

In some embodiments, especially those where liquid or gas permeabilitythrough plastic is an issue for long-term storage, the holding componentwith the plugs inside can be frozen.

The holding component is ideal for storing reagents in a controlledenvironment. The holding component can be used to dispense the plugsinto another component in the microfluidic system or can be used todispense the plugs into a volume or onto a surface (such as a well plate(e.g. commercially available 96-, 384- or 1536-well plates), microscopeslides, cell culturing media, etc). If the plug fluid has a low surfacetension (like a fluorocarbon), then the plugs can be dispensed simply bydripping the plugs from the holding component. To facilitate dispensingof single plugs, markers or spacers could be used to aid the user invisualizing single drops. In addition, as the dispensing is takingplace, spacers may facilitate the break up of the fluid into partscontaining the individual plug fluids. A volatile carrier fluid may beused so the carrier fluid is removed by evaporation. Examples ofvolatile carrier fluids include butanes, perfluorobutanes andperfluoropentanes.

Holding components are ideal for preparing crystallization experiments.For microbatch experiments, the preloaded cartridges may containprecipitant plugs of varying concentrations. These plugs can be mergedwith a stream of protein solution to set up crystallization screens. Forvapor diffusion experiments, preloaded cartridges of alternating plugs(precipitant alternating with desiccant)) may be used. For vapordiffusion experiments, the system can be configured such that only theprecipitant plugs will merge with the protein stream. To conduct freeinterface diffusion experiments, instead of merging the protein streamwith the plugs of varying reagent, the protein streamforms its own plugswhich, after the flow is stopped, are allowed to come into contact withadjacent precipitant plugs, forming the free interface. Holdingcomponents are also ideal for conducting assays, reactions, screens,etc.

In order to further prevent the merging of the plugs in the preloadedcartridge, each pair of plugs can be separated by a spacer (as describedbelow).

Each plug in the linear array of fluid plugs in the holding componentcan comprise at least one solvent and at least one reagent. Mixtures ofsolvents and reagents can be used. In some embodiments, each plugcomprises a different concentration of the same reagent or mixture ofreagents. In other embodiments, at least two plugs in the linear arraycomprise different reagents. The components of a linear array can bechosen to span a certain portion of chemical space. As used herein,chemical space encompasses all possible small organic and inorganicmolecules and materials, including those present in biological systems,at all possible concentrations. A linear array can span a portion ofchemical space when the plugs comprising the array are chosen so thatthey share a similar feature (such as similar molecular masses,lipophilicities, topological features, reactive groups, core structures,etc.). Compounds with similar features have already been grouped intovarious existing databases (see e.g., Dobson, Nature, 432:824-828 (2004)and Feher, J. Chem. Inf. Comput. Sci. 43:218-227 (2003)).

A linear array of fluid plugs suitable for conducting a crystallizationscreen may be chosen. For protein crystallography, arrays with different“sensitivities” can be used. That is, for a coarse screen, the array maycontain plugs with different reagents. Refinement screens may containplugs with reagents with related properties or different concentrationsof the same reagent.

Markers can be used to index the linear array as described in co-pendingapplication Ser. No. 10/765,718, incorporated herein by reference. Incertain embodiments, every other plug in the linear array comprises atleast one solvent and reagent, and the remaining plugs comprise at leastone solvent and at least one marker.

In addition to using fluid plugs as markers, marked spacers could alsobe used as indexing markers. Such spacers could be colored, or couldcontain a component that was detectable by other means.

In other embodiments, markers may be injected at greater intervals, forexample after about every 5, 25, 50, or 100 plugs. The use of markerscan be combined with the use of a measuring component. Such acombination can facilitate aligning the measuring component marks withthe array, and small variations of volume injected and any variations inthe diameter of the receiving component would not induce significanterrors (that is, the errors will not accumulate), because the measuringcomponent can be readily realigned with the receiving component.

The holding component contains two or more fluid plugs; typically atleast about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 1000, 2000, 3000 or10,000 fluid plugs.

The concentration of reagent in the fluid plug will depend on the goalof the reaction. For crystallization experiments, each plug in thelinear array of fluid plugs typically comprises about 1 pL to 10 μL,preferably about 1 to 50 nL of plug fluid. For crystallizationexperiments, each plug in the linear array of fluid plugs typicallycomprises less than about 10 M of reagent. For some reactions, the plugsmay contain reagent with a concentration less than about 1 fM. For otherreactions, such as screening for nanomolar or femtomolar inhibitors forenzymes or ligands for proteins, the solutions in the plugs will be muchmore dilute. For applications other than crystallography, other volumesand concentrations may be used. For example, each plug in a linear arraycan comprise about 0.001 fL to 10 mL of plug fluid.

Combining Component

The combining component can be used (a) to merge plugs with a stream ofreagent fluid, (b) to merge plugs with other plugs, (c) to formalternating plugs by injecting a stream of reagent fluid or a second setof plugs between a first set of plugs. In (a) and (b), the merged plugsmay be homogenous or may be a drop-inside-a-plug or aplug-touching-a-plug. In (c), the alternating plugs may later merge, ifmerging is induced as described below. Additionally, a first set ofplugs comprising a first fluid can be merged with a stream comprised ofa second fluid that is immiscible with the carrier fluid, the firstfluid and, if present, the spacer. Using this method, each plug in theresulting set of plugs will contain a section of the second fluid thatdivides the first fluid and is in direct contact with the two resultingsections of the first fluid.

The combining component may use active on-chip and off-chip components(such as valves, etc) to control the flow. Simple combining componentsthat contain only or predominantly passive components would be preferredfor some applications. Some of the passive combining components aredescribed below.

In some embodiments, the combining component comprises at least onedelivery microchannel, which is suitable for delivering an additionalreagent to a plug without significant formation of new plugs containingjust the additional reagent. In other embodiments, such as systems forfree interface diffusion crystallization experiments, new plugscontaining the additional reagent are formed in the combining component.

In one embodiment, the combining component may use a stream of thereagent fluid merged through at least one channel. Various methods ofinducing and controlling fluid flow may be used. For example, the streamof reagent fluid may be driven by a method that generates constantpressure. The pressure may be chosen such as it is lower than thecapillary pressure of the reagent stream entering the carrier fluid. Ifthis is the case, the stream will not enter the carrier fluid and willpreferentially inject into the plugs. The capillary pressure of thereagent stream may be increased by decreasing the diameter (at thejunction) of the channel through which the reagent stream is applied. Inaddition, it may be increased by increasing the surface tension at thecarrier/reagent stream interface. If the flow rate is controlledvolumetrically, for example by a syringe pump, then a similar effect maybe obtained by introducing an elastic element between the pump and themerging junction. Preferably, capillary pressure is sufficient to changethe capacity of the elastic element by approximately the volume of thereagent stream desired to be injected into the plug. The elastic elementmay be, for example, a gas bubble (which would be compressed by thecapillary pressure) or a flexible element of the microchannel (such asan elastomeric part allowing for expansion of the microchannel).

In certain embodiments, the combining component can be any microfluidicdevice which allows the array of plugs to be merged with either (1) astream of reagent (such as a stream of protein) or (2) a stream ofreagent plugs. In either case, once the array of plugs passes throughthe microchannel of the combining component, at least one merged plugwill result. Reagent is transferred into at least one plug in an arrayas the array passes by a delivery channel in fluid connection with themicrochannel. The merged plugs then flow through a downstream channelwhere they can be further manipulated, mixed, monitored, sorted,collected, etc. In other embodiments, such as systems for free interfacediffusion crystallization experiments, the combining component can beany microfluidic device which allows the array of plugs from the holdingcomponent to be combined with (1) a stream of reagent (such as a streamof protein) or (2) a stream of reagent plugs in such a way that a newlinear array of plugs is formed in which the original plugs from theholding component alternate with new plugs containing the introducedreagent.

FIG. 3a illustrates a combining component comprising multiple deliverychannels 14 configured to be substantially parallel in relation to eachother, each delivery channel perpendicular to and in fluid communicationwith a microfluidic combining channel 15. The delivery channels branchfrom a common inlet channel 16 and due to their relatively short lengthact in concert. In this embodiment, an array of plugs in the holdingcomponent can be delivered to the combining component through one end ofthe microfluidic combining channel 15. A reagent solution (such as anaqueous protein stream) is delivered to the inlet channel 16. As plugspass by the delivery channels, the reagent solution is delivered to theplugs. The merged plugs flow through the combining component and can becollected in a second holding component or delivered to a receivingcomponent for further manipulation.

Channels with a narrowed junction can be used to reduce potentialproblems of cross-contamination of the introduced reagent stream byreagents in the array of fluid plugs during merging. For example, inprotein crystallization experiments in which a protein stream isintroduced to an array of fluid plugs containing a variety ofprecipitation reagents, it is desirable to avoid cross-contamination ofone precipitant from one plug to another upstream plug via contaminationof the protein stream. The use of a smaller delivery channel limits theamount of reagent that can be added to each plug, however. To enable theintroduction of larger quantities of reagent, while maintaining theadvantages of narrowed junctions, merging junctions can be fabricatedwith split multiple channels. In comparison to a single channel with anidentical total cross-sectional area and volumetric flow rate, thedivided channels have a significantly higher shear rate than a singlewide channel, thereby reducing contamination by diffusion. The smallerchannels will also act to reduce any surface tension driven convection.Such multiple small junctions can also be used to aid the selectivemerging process in vapor diffusion.

Channels may act in concert when the capillary pressure of the streambeing delivered through a delivery channel is approximately equal to orlarger than the pressure drop across the delivery channels. If a plugcomes into the proximity of one of the delivery channels, and thereagent stream merges with the plug, the reagent stream will flow intothe plug. The pressure due to surface tension at the carrierfluid/merging stream interface will resist the flow of the reagent intothe carrier fluid through other delivery? channels. For example, ifthree delivery channels are used, and the reagent stream is flowing onlythrough one of the channels, the pressure due to surface tension at thecarrier fluid/merging stream interface at the other two channels shouldbe sufficiently high to be able to sustain the pressure equal to thepressure drop due to the flow of the reagent fluid through the onechannel. The pressure due to surface tension at the carrierfluid/merging stream interface depends on the geometry of the junctionbetween the delivery channels and the combining channel. The pressuredrop depends on the length of the delivery channels, theircross-sectional dimensions, the viscosity of the reagent fluid, and theflow rate. Therefore, many parameters may be adjusted to achieve thedesired relationship between the pressure due to surface tension at thecarrier fluid/merging stream interface and the pressure drop.

Multiple delivery channels and the spacing between the plugs may bedesigned such that there is always at least one plug in the vicinity ofa delivery channel. This configuration enables consistent merging of thestream with plugs, without the injection of the reagent stream into thecarrier fluid. In addition, such a configuration may be used to achieveselective merging, where only some of the plugs merge with the reagentstream, for example only the first type of plugs merges with the stream,while the second type does not. Selective merging may be achieved whenthe second type of plugs have a lower tendency to merge with the streamof reagent (this selectivity may be achieved, for example, by increasingthe viscosity of the second type of plug fluids, or by decreasing thesurface tension at the second plug fluid/carrier interface). Inaddition, polymers may be added to the second plug fluids, especiallysurface active polymers that would increase interfacial viscosity. Toachieve selective merging, this configuration may be used especiallywhen the delivery channels and the spacing between the plugs is suchthat there is always at least one plug of the first type in the vicinityof the delivery channel.

FIG. 12 shows an example of another possible merging geometry that isalso designed to minimize cross-contamination. In this geometry, a smallside channel is used to direct the carrier fluid to assist to break offthe reagent stream.

In some applications, it may be preferable to drive the flows into thecombining component using a simple method, for instance one that is notcapable of providing substantially constant pressure or constant volumeflow to one or more inlets. Some methods may be designed so they do notrequire electric power or computer control. Such methods would be usefulfor in-the-field diagnostics and testing, and for other applicationswhere simplicity and low cost are desirable. For example, in thecombining component, the relative flow rates of the plugs through theholding component and the stream of reagents to be combined must becontrolled. This control may be easily achieved with multiple syringepumps or other established methods of pumping. In addition, one may usea fluidic rationing device. This device would be placed in between apressure source, such as a syringe or a pump, and the fluid streams orarrays to be pushed. This device may be designed as a Y-junction, withthe inlet corresponding to the bottom of the Y, and the two outletscorresponding to the top of the Y. The device may be filled with acarrier fluid or carrier fluids, and the outlets connected to the inletof the holding component and the inlet corresponding to the reagentstream of the combining component. The ratio of fluidic resistances ofthe two outlets may be configured to correspond to the desired ratio offlow rates. The individual fluidic resistances of the two outlets couldbe controlled by individually varying the relative lengths and/ordiameters of the two arms of the Y. Preferably the fluidic resistance ofthe two outlets of the Y are significantly higher (for example by afactor of 100) than the fluidic resistances of the holding and combiningcomponents, so that the flow rates through the holding and combiningcomponents are determined by the ratio of fluidic resistances of the twoY outlets. An analogous approach may be used to control flow rates ofmore than two streams. This approach is attractive because, in manyapplications, as long as the capillary number is in the correct range,the plug behavior is determined by the ratio of flow rates, rather thanthe absolute values of flow rates. In this approach the flow may bedriven by a single pump or by hand, and still provide the correct ratiosof multiple flow rates. Such a fluidic rationing device could be offeredin a kit with a holding component.

Any volume of reagent solution can be merged with a fluid plug. The flowrate of the reagent solution can be varied during combining to vary theconcentration of reagent in the merged plugs. The amount of reagentdelivered is essentially determined by the intended detectiontechnology. For example, femtoliter amounts (or less) of reagentsolution can be delivered if the reaction product can be detected.

For protein crystallization, typically the volume of protein solution tobe combined with the plug fluid is similar to the volume of the plugfluid (typically a 1:5 to 5:1 ratio of protein solution to plug fluid isused). The amount of the protein solution combined with the plug can becontrolled by the ratio of the flow rates of the plug stream and theprotein stream. Again, the amount of protein solution delivered willdepend on the goal of the experiments. If the goal is simply to detectthe formation of crystals or precipitates, pL volumes can be used. Ifthe goal is to obtain a crystal structure, 10 nL or more may benecessary. In general, about 1-100 nL, preferably about 10 nL, ofprotein solution is delivered to each plug. For protein crystallography,typically the concentration of protein solution is about 0.01 to 100mg/mL, preferably 0.1 to 50 mg/mL.

Receiving Component

The receiving component comprises a first microchannel with at least oneopen end configured for making a fluid connection with a combiningcomponent or other microchannel. The receiving component can be amicrofluidic device such as that described elsewhere in thisapplication. The receiving component can be constructed identically tothe holding component described above. When used to conduct proteincrystallography experiments, it may be useful for the receivingcomponent to be constructed of a material, such as glass, suitable foron-component x-ray diffraction of crystals formed in plugs. When usedfor assays, the receiving component may be constructed of a materialsuitable for optical detection such as glass or plastic.

The receiving component can comprise a device to control the temperatureof the contents of the plugs, for example, in order to control the rateof reactions. The receiving component could also comprise a device forilluminating or irradiating the contents of the plugs, for example toinitiate photochemical reactions. The receiving component can bedesigned to sort or monitor plugs. The receiving component may beseparate from or integrated with devices for detecting or analyzing thecontents of the plugs.

The receiving component may comprise an active surface. This activesurface may induce wetting by the plug fluids. Wetting by plug fluidsmay be used, for example, to create contact between plug fluids, forexample in macromolecular crystallization by free-interface diffusion.This active surface may also be reactive. The reactions may occur uponcontact with plug fluids, and/or by transfer of the reagents between theplug fluid and the surface (in either or both directions) through thecarrier fluid, spacers, or markers.

Measuring Component

The measuring component allows the user to identify individual plugs bytheir location within the microchannel. A measuring component may beuseful if the number of plugs in a linear array is too large to allowmanual counting of the plugs. It may also be useful if automatedcounting of plugs is desired.

The receiving component can comprise an integrated measuring component(such as one or more physical marks) for identifying the placement of afluid plug within the microchannel. Alternatively, a measuring componentseparate from the receiving component can be used. When a separatemeasuring component is used, it is preferable to provide a kitcontaining the measuring device and the receiving component. In certainembodiments, a kit can comprise a holding component and a matchedmeasuring component designed to indicate the location or locations ofthe contents of the holding component after they have been moved to thereceiving component. The measuring component could, for example, be acard designed to fit onto a receiving component.

The measuring component can be a ruler, where the measurement markingscorrelate to the location of plugs within an array in a receivingcomponent. The measurement markings can be formed by dots, lines,segments, etc. The markings can be luminescent (fluorescent,phosphorescent, chemiluminescent, etc), reflective or absorbent, basedon differences of refractive index, and other form of optical marks.Markings can also be magnetic, charged, topographic, etc.

The markings could be numbers as well. The markings can behuman-readable, and could include words, or can be machine-readable, orboth. Possible machine-readable markings include barcodes, implemented,for example, using any of the methods above, and also includingnano-scale barcodes as described in Nicewarner-Peña, et al., Science294: 137-141 (2001).

The measuring component can aid visualization when the plugs aredifficult to locate visually, as may occur when the carrier and plugfluids have matched refractive indices. Refractive index matching can behelpful for visualizing the contents of plugs.

A measuring component need not mark the location of every plug. It couldsimply mark multiples of plugs, e.g. every third plug or every fifthplug, or could mark the location or locations of significant changes inreagent composition along the linear array of plugs.

To aid the user, the array of plugs could contain marker plugs orspacers. The marker plugs or spacers could then be used to allow theuser to initially align the measuring component with the array, or tohelp the user identify the plugs in the receiving component without theuse of the measuring component. Several types of markers may be used.For example, a marker of the first type every tenth plug, a marker ofthe second type every fiftieth plug, and a marker of the third typeevery 250^(th) plug, etc. The different types of markers can differ, forexample, by color.

Formation of Components

Components comprise microchannels that can be formed using methods knownin the art. For example, pending application Ser. No. 10/765,718describes the manufacture of microchannels and use thereof forsplitting, merging, and otherwise manipulating fluid plugs.

Components of this invention can be formed from capillaries, tubing witha diameter of less than about 10 mm, preferably less than about 1 mm,substrates with etched, or molded, or embossed microchannels, orcombinations thereof. Components may contain multiple channels, eitherjoined together or separated. Preferably, the microchannels have acircular or rounded cross section, rather than a rectangular crosssection, to facilitate transport of plugs and reduce coalescence ofplugs. However, any shape can be used.

To fabricate channels with circular or nearly-circular cross-section,thin pieces of tubing or capillaries may be inserted into rectangularchannels. This technique may facilitate formation of plugs, and also mayprovide a method of connecting chips with other components. Multiplepieces of tubing may be inserted into multiple channels. The tubing orcapillaries may be chosen such that they are preferentially wetted bythe carrier fluid. For example, Teflon is preferentially wetted byfluorinated fluids over aqueous fluids. The tubing or capillaries mayoptionally have flared, funnel-like ends (or narrowed ends) tofacilitate connections with other components. To fabricate such devices,the tubing or capillary may be either inserted into the channel beforesealing of the layers (such as the sealing of PDMS layers), or after therectangular channel is fabricated. It may also be desirable to fill thespace between the outer surface of the tubing or capillary and therectangular channels. This space may be filled with a viscous liquid, ora polymer. PDMS may be used, for example.

Individual components can be formed from the same or differentmaterials. Components can be formed from a material that responds tocontact with water. Polymers have been synthesized that respond tocontact with water (Senshu et al., Langmuir, 15, 1754-1762 (1999); Moriet al., Macromolecules 27, 4093-4100 (1994)). These diblock polymerscontain hydrophilic and hydrophobic domains. When exposed to air, thehydrophobic block migrates to the surface to minimize the surface energyand the surface is hydrophobic. Upon contact with water, surfacereconstruction occurs and the hydrophilic block migrates to thepolymer-water interface until the water is only in contact with thehydrophilic block. Exemplary polymers include poly(styrene-b-2,3-dehydroxypropyl methacrylate) and poly(4-octylstyrene-b-2,3-dehydroxypropyl methacrylate).

Opaque particles such as carbon black particles (preferably those withdiameters smaller than about 5 microns) can be incorporated into thematerial used to make the components. In this manner, substantiallyblack channels can be fabricated, eliminating the problems associatedwith background light (e.g. increasing the sensitivity of fluorescentanalysis).

An adapter may be used as a connection, for example to connectcapillaries or pieces of tubing. The adapter may be a short piece ofglass tubing with the I.D. (inner diameter) narrowing in the middle.Adapters of different I.D's are commercially available. The surfacechemistry of the inside wall of glass adapters can be made hydrophobicby silanization. Adapters can also be fabricated easily from stretchedTeflon tubing. To do this, a piece of Teflon tubing is stretched at apoint. Both the I.D. and O.D. (outer diameter) of the stretched pointwill decrease resulting in tubing suitable for the use as an adapter.

Plug Formation

As used herein, “plugs” are formed when at least one plug-fluid isintroduced into the flow of a carrier in which the plug fluid isimmiscible. The flow of the fluids in the microfluidics device isinduced by a driving force or stimulus that arises, directly orindirectly, from the presence or application of, for example, pressure,radiation, heat, vibration, sound waves, an electric field, or amagnetic field. Plugs may vary in size but, when formed, theircross-section is typically similar to the cross-section of the channelsin which they are formed. In non-circular shaped channels, the plugsshould be similar in cross sectional size to the cross section of atleast one-dimension of the channel. The plugs should be substantiallysurrounded by carrier. That is, the carrier fluid should wet themicrochannel to a greater extent than does the plug fluid. Thecross-section of a plug may change if it passes into downstream channelsof differing diameter or when it is merged with additional fluid. Plugsmay vary in shape. The term “plugs” also includes plugs-within-plugs,drops-within-plugs and solids (such as beads)-within-plugs. An exceptionto this general definition is that a plug can be smaller than the crosssection of a microchannel when spacers are used.

“Merged plug” as used herein refers to either (1) the combination of twoor more plugs to form a single plug or (2) the juxtaposition of twoplugs in a microchannel so that they will merge with time.

In order to form and move plugs through a microchannel with minimaldispersion, the fluid inside the plugs must not adhere to the walls ofthe microchannel. To achieve this, the surface tension at theplug/channel interface must be higher than the surface tension at theplug/carrier interface. When the plugs are aqueous and the carrier is anoil, the dimensionless capillary number Ca is ideally less than about0.1:Ca=Uμ/γwherein, γ [N m−1] is the surface tension at the oil/water interface, Uis velocity in m/sec and μ is viscosity (Pa·sec).

The microchannels used in the present invention can be surface treatedto make them wettable by the carrier fluid preferentially over wettingby the plug fluid. For example, a glass capillary can be silanized todecrease the wetting of aqueous plugs versus a carrier. In addition, amicrochannel can be initially flushed with carrier prior to introductionof plugs.

Plugs can be formed from essentially any fluid such as from one or moreaqueous solutions, one or more organic solutions, one or more inorganicsolutions, or mixtures thereof. Plugs can be formed from water, stocksolutions, buffers, reagents, solvents, salt solutions, polymersolutions, precipitant solutions, metal suspensions, cell suspensions,or the like.

The size of the plug can be controlled as described throughout thisapplication; for example, by the relative volumetric flow rates of theplug fluid and carrier into the microchannel. Volumetric flow rates arepreferably controlled using pumps.

Various processes may occur during formation of plugs. For example,formation of lipidic cubic phases (LCP) may occur when a stream of alipid (such as MO, monoolein) is combined into a plug with an aqueoussolution, and the two liquids are vigorously mixed inside a plug. Anaqueous solution of a macromolecule, such as a protein or a membraneprotein, may be used. This mixing may be performed by chaotic advection,induced when a plug is traveling through a plurality of bends in awinding channel (as previously described by the present inventors).Viscous carrier fluid may be used to induce higher shear inside theplugs, to ensure more effective mixing. This method is attractive, forexample, for miniaturizing crystallization in LCP.

The spacing between plugs in microchannels can be controlled in a numberof ways. In one embodiment, one set of reagent solutions is continuouslyinjected into a flow of immiscible carrier fluid to form plugs and asecond set is continuously injected further downstream. If carrier fluidwith suitable surfactant is used, the second set of solutions willprefer to form plugs that are dispensed directly adjacent to plugs thatare already formed, rather than to inject into them. The plugs are keptfrom coalescing by surfactant assemblies at the interfaces between thereagent solutions and the carrier fluid, and by a layer of the carrierfluid between the two plug fluids. Pairing up of plugs in the mainchannel would occur if plug fluids have different properties, forexample different viscosities. This pairing up may be useful forapplications, for example for crystallization of macromolecules usingvapor diffusion and in free-interface diffusion methods.

Plugs of alternating composition can be formed by flowing aqueoussolutions head-on into a stream of carrier. The flowing carrier providesa barrier between the plugs that prevent them from coalescing. Formationof alternating plugs is controlled by the capillary number. Formicrochannel inlets of the same width, the ratio of the flow rate of theaqueous streams will determine the ratio of the size of the droplets. Ifthe flow rate of the top aqueous stream is two times faster than theflow rate of the bottom aqueous stream, then the size of the resultingdroplets will then be in an approximately 2:1 ratio. In some cases,plugs of approximately the same sizes may form, but two plugs from thefaster stream may form for every one plug of the slower stream. Theratio of the dimension of the microchannel inlets can also be designedto affect the size ratio of the droplets. If the microchannel for thetop aqueous stream is two times wider than the microchannel for thebottom aqueous stream, then the size of the resulting droplets may be inan approximately 2:1 ratio. Pairing up of alternating plugs in the mainchannel would occur if plug fluids have different properties, forexample different viscosities.

The concentrations of solutions of the plugs along the array can berapidly varied simply by changing the relative flow rates of the plugfluid stream components. This principle can be used to set up lineararrays of plugs in a microchannel. For example, it can be used to set upcrystallization trials by forming varying concentrations of proteinand/or varying concentrations of precipitating agent within an array ofaqueous plugs. Preloaded cartridges containing such linear arrays can beused for optimization of crystallization conditions.

In some embodiments it may be simpler or less expensive to form plugslarger than the plugs required for an application. For example, it maybe straightforward to make reliably larger plugs of about 1 microliterin volume, rather than smaller 10 nL plugs desirable for proteincrystallization screening. If a loading component is available forcontrolling larger volumes of fluid, it may be preferable to connectmultiple holding components to one loading component, and fabricate inparallel multiple holding components with smaller plugs. For example, ifa robotic loading component is available to aspirate 1 microlitervolumes, and 25 capillaries are connected in parallel to this loadingcomponent so that equal volumes are aspirated into each capillary, thenby successfully aspirating 1 microliter volumes of plug fluids, carrierfluids, and spacers, an array of capillaries would be created that wouldcontain smaller approximately 40 nL plugs, separated by approximately 40nL of carrier fluid and approximately 40 nL spacers. The robotic loadingcomponent could, for example, aspirate in sequence carrier fluid, aspacer (air, for example), carrier fluid, a first reagent composition,carrier fluid, a spacer, carrier fluid, a second reagent composition,carrier fluid, a spacer, carrier fluid, a third reagent composition,carrier fluid, etc. The capillary pressure for the solution entering theholding components is preferably significantly lower than the pressure(or pressure differential) used to drive the flow. This would ensurethat the fluids are entering all holding components to a substantiallyequal extent. In addition, the entrance into the holding component maybe designed so that there is an increase of the capillary pressure asplug fluids enter and proceed into the holding component (in oneembodiment, this increase may be achieved by using a holding componentthat narrows from the point of entrance). Such an entrance mayfacilitate more equal distribution of volumes entering the holdingcomponent. Preferably, the holding components are not wetted by the plugfluid, to prevent plug fluids adhering to the holding components andcausing cross-contamination.

This approach described above is not limited to aspiration into multipleholding components, and is also useful for other methods of dispensing.

Plugs can also be made by splitting larger plugs into smaller ones. Inthis embodiment, a microchannel is split into one or more smallerreceiving microchannels at a single junction. The junction is fabricatedto be narrow such that the capillary number is increased. The process ofsplitting plugs can be controlled by adjusting the pressures or pressuredrops in the receiving channels. The receiving channels are preferablydesigned such that the pressures due to surface tension at the outletsare significantly lower (e.g. 10 times, preferably 100 times or 1000times or more) than the pressure drops across the receiving channels.This ensures that splitting is controlled by the pressure drops acrossthe channels rather than by the fluids that are being dispelled at theoutlets. The capillary pressure at the outlets may be reduced bypre-filling the holding component with the carrier fluid, and immersingthe outlet of the holding component into a reservoir with a fluid thathas low surface tension with the carrier fluids, the plug fluids, andthe spacers. Also, the capillary pressure at the splitting junctions ispreferably significantly lower than the pressure drops across thereceiving channels. Also, the capillary number at the splitting junctionshould be high enough to allow for splitting to occur.

Alternatively, a long plug could be flowed into a stream of carrierfluid at a junction. The long plug breaks up into a series of smallerplugs separated by the carrier fluid.

Large plugs can be broken up into smaller plugs such that the smallerplugs contain different concentrations of the solution originallycontained in the large plugs. When applied to protein crystallization,this method allows one to perform a sparse matrix screen and a gradientscreen with the same array. In one embodiment, a holding componentcontains large (e.g., about 100 nL) plugs of the appropriate solutionsfor a sparse matrix screen. These large, long plugs are flowed into astream of carrier fluid such that they break up into a series ofsmaller, shorter plugs. The flow rate of the large plugs relative to theflow rate of the carrier fluid controls the size of the smaller plugs.Protein solution is subsequently combined with the smaller plugs througha peripheral channel downstream, and the flow rate of this peripheralstream relative to the flow rate of the stream containing plugs controlsthe concentration of protein in the smaller plugs. A buffer solution maybe similarly added to the small plugs to insure that eachcrystallization assay is performed at an identical volume. Spacers(e.g., gas bubbles) may be contained in the holding component betweenlarge plugs, and may be introduced additionally during the manipulationsof plugs.

Alternatively, the large plugs may be flowed into an aqueous stream.This stream may contain one or more solutions. When applied to proteincrystallization, the stream may contain a laminar flow of protein andbuffer solutions. The resulting multi-component stream is flowed into astream of carrier fluid where the combined aqueous stream breaks up intoa series of plugs. Again, in this application, the large droplets maycontain the solutions for a sparse matrix screen and the flow rates ofthe aqueous streams and large plugs may be varied to createconcentration gradients of any of these reagents, resulting in a hybridscreen. Spacers may be used in the holding component between largeplugs. Markers may be also used between large plugs.

Carrier Fluid

The carrier can be any liquid or gas that is substantially immisciblewith the plug fluid. Preferably, when the plug is aqueous, the carrieris liquid. The surface tension of a plug fluid in a carrier is ideallybetween about 5-15 mN/m, preferably about 10 mN/m. Other non-zero valuesof the surface tension may be used. The carrier can be permeable to theplug fluid or reagents.

For membrane protein crytallography, perfluoroamines or their mixturesare ideal carriers, because they preferentially wet surfaces (especiallyfluorinated surfaces in Teflon devices) over the aqueous plug.

In some embodiments, it is desirable add a surfactant into the carrierfluid. Surfactant may be used to control the surface tension and thewetting properties of the carrier fluid. We have used1H,1H,2H,2H-perfluorooctanol as the surfactant when fluorinated fluidswere used as carrier fluids. Surfactants may also be used to controlnon-specific adsorption of the contents of the plug to the interfacebetween the plug and the carrier. For example, fluorous-solublesurfactants can be used to control non-specific protein adsorption at afluorinated carrier-aqueous interface. An exemplary fluorous solublesurfactant is the oligoethylene glycol molecule triethyleneglycol mono[1H,1H-perfluorooctyl] ether CF₃(CF₂)₇CH₂O(CH₂CH₂O)₃H). This surfactantarranges itself at the fluorinated carrier-aqueous interface during plugformation and presents a monolayer of oligoethylene glycol groups. Thisinterface resisted the adsorption of a wide variety of proteins andenzymes, including a concentrated solution of fibrinogen, a proteinknown to adsorb quickly and strongly to surfaces. Surfactants with thesame ability to resist non-specific adsorption can be extracted fromcommercially available Zonyl FSO-100 from DuPont via fluorous-aqueousextraction. Other oligoethylene glycol congeners with variablefluorinated alkane chain lengths, variable glycol lengths, and variousspacer links also prevent protein adsorption at the aqueous-fluorousinterface. Conversely, surfactants capped with non-inert functionalgroups can attract and bind proteins to the liquid-liquid interface.

Spacers

The arrays of the present invention can optionally contain spacers.Suitable spacers are composed of at least one liquid (e.g., ionicliquids, fluorosilicones, hydrocarbons, and fluorinated liquids), gas(preferably an inert gas such as nitrogen, argon or xenon), gel or solid(e.g., polymers such as polystyrene) that is immiscible with both theplug fluid and the carrier. The arrays of the present invention cancontain multiple types of spacers.

Spacers can also contain markers so they can be used to index plugs asdescribed above. Spacers may also be used to reduce cross communication(e.g. by preventing optical communication or by preventing permeability)between plugs. Spacers may also have functional properties—for example,when xenon bubbles are use as spacers in macromolecular crystallography,especially when experiments are conducted under increased pressure,xenon may incorporate into a growing macromolecular crystal, improvingits diffraction properties.

The spacer can be formed and manipulated using the similar methodsdescribed for formation and manipulation (e.g. splitting) of plugscomposed of a liquid. In particular, a stream composed of both liquidplugs and gas bubbles may be formed using the same methods used to formstreams of plugs of alternating liquid compositions. Spacers may beintroduced during the robotic fabrication of the array. If an array oflarger plugs separated by spacers, is split to fabricate several arrayof smaller plugs, then spacers are preferably also split.

Spacers can play an important role in manipulations of plugs. First, ifundesirable merging of plugs occurs, spacers can be inserted between theplugs to minimize merging. Such spacers may allow transport of an arrayof plugs through longer distances than without the spacers. Such spacersmay also facilitate transfer of plugs in and out of devices andcapillaries (or transfer through composite devices made of combinationsof devices and capillaries).

Spacers may also be used to facilitate free interface diffusion inside aplastic or PDMS channel. With a gas bubble in between plugs containingreaction components, by applying pressure from both ends, the gas bubbleis purged out through the plastic or PDMS wall. The two plugs are thenbrought close enough to allow diffusion across the newly formedplug-plug interface. Such a system may be applied to proteincrystallization, where one plug contains protein and the other containsprecipitating agent. If pressure is applied, and the gas bubble iscompressed, the plugs separated by the spacer may come into contact.

Kits

As discussed above, one embodiment of a kit comprises a receivingcomponent and a measuring component.

In another embodiment, a kit of the present invention can comprise acombining component, a reservoir and a pump. A pump suitable for use inthis embodiment is one which can provide sufficient pressure to drive asolution, for example, a protein solution, from the reservoir throughthe inlet, through the delivery channels, and into the microchannel ofthe combining component.

In another embodiment, a kit comprises 2 or more holding components. Forexample, the holding components in the kit could span a portion ofchemical space. In another embodiment, the kit could comprise multipleholding components spanning a genome.

In another embodiment, a kit comprises a combining component and one ormore receiving components, wherein the microchannels of the combiningand receiving components are sized and shaped so that they can form afluid connection with each other. The kit can also contain two, three,four, five, ten, twenty, fifty or more receiving components.

In another embodiment, a kit comprises one or more holding componentsand a combining component, wherein the microchannels of the combiningand holding components are sized and shaped so that they can form afluid connection with each other.

In another embodiment, a kit comprises one or more holding components,one or more combining components, and one or more receiving components,wherein the microchannels of the combining, holding and receivingcomponents are sized and shaped so that they can form a fluid connectionwith each other.

In another embodiment, a kit comprises one or more loading components,one or more holding components, one or more combining components, andone or more receiving components, wherein the microchannels of theloading, combining, holding and receiving components are sized andshaped so that they can form a fluid connection with each other.

Transfer of an Array of Plugs into Various Components

The plugs can be transferred from one component into another byinserting one end of a component directly into one end of the othercomponent and sealing the junction to inhibit leaking. Alternatively, anadapter (such as a funnel) can be used to connect the variouscomponents.

The plugs may be pushed from the microchannel of a first component intothe microchannel of a second component by applying a pressure to the endopposite the outlet of the first component or may be pulled into thesecond component by applying a vacuum inside the channel of the secondcomponent. Other standard methods of driving fluid flow can also be usedas described above.

The rate of mixing of various fluids in a plug can be controlled asdescribed in copending application Ser. No. 10/765,718. For example,when plugs are transported through winding channels, rapid mixing can beinduced. To increase the reaction time in plugs that are transferredfrom the loading component into the holding component or from theholding component to the combining component, or from the combiningcomponent to the receiving component, they may be either transportedthrough at a sufficiently low velocity, or with flow completely stoppedor significantly reduced.

In some applications of the present invention, it is desirable to allowa plug to be combined with more than one subsequent plug or stream,either in immediate sequence or with some delay between the combiningevents. In some embodiments, the combining component can contain morethan one delivery channels connected to a corresponding number ofreservoirs, allowing an array of plugs to pass by the delivery channelssequentially. Reagent from each delivery channel is thus transferredinto each plug as it passes by each inlet. In such an embodiment, delaybetween each transfer of reagents can be controlled by the distancebetween the delivery channels and the flow rate of the array of plugs.In other embodiments, a receiving component containing an array of plugspreviously combined with a first reagent using a combining component canbe subsequently combined with a second reagent using a combiningcomponent. The combining component used in combining with the firstreagent may be either the same device or a different device than thatused in combining with the second reagent. For example, an array ofplugs containing a library of small molecules which are potentialinhibitors of an enzyme can be first combined with an enzyme solutionand allowed to equilibrate. Subsequently, this array of plugs containingenzyme and small molecules can be combined with the enzyme's substrate,and the enzyme-catalyzed reaction can be monitored to determine theefficacy of the potential inhibitors. The use of tubing withapproximately circular cross-section is especially useful for achievingreliable transport of plugs over long distances, and it may be alsoemployed to achieve appropriately long delays for reactions.

Components can be reused. For example, the outlet of a loading componentcan be connected to a first holding component (such as a capillary) tocollect an array of plugs. After transfer of the array, the firstholding component can be taken off the loading component and, ifnecessary, sealed or capped. A second holding component can then beconnected to the loading component and filled. Combining components maybe reused in a similar fashion.

Automated devices can be used to interface multiple loading and holdingand receiving components. For example, one loading component may be usedto load plugs into many holding components, which are switched intoplace by an automated piece of equipment. During incubation, the holdingor receiving components may be transported to a monitoring station thatwould monitor the extent of reaction or crystallization in plugs. Theholding or receiving components may be continuously transported past themonitoring station or several stations. This monitoring could be used,for example, to detect a plug or a series of plugs in whichcrystallizations or reactions took place. The receiving component couldbe used, for example, to detect, sort and/or separate plugs in whichdesired processes took place.

FIG. 1 illustrates a funnel shaped adapter 2 that can be used to make aconnection between holding and loading components. In this illustration,the holding component is a capillary 3 and the loading component is amicrofluidic device 1. Alternatively, the adapter can be attached eitherto the microfluidic device or the capillary.

FIG. 2a illustrates how a single array of larger plugs can be split intomultiple arrays of substantially identical character, each containingsmaller plugs. This splitting is achieved by continuous directing theinitial fluid flow into pairs of smaller sized channels of substantiallysimilar sizes. Such a device can be used to split arrays ofmicroliter-size plugs in a microfluidic device with large (for example,˜800 μm) channels into nL plugs in smaller channels.

Reactions within Plugs

The present invention provides a system for plug-based microfluidicmanipulation of small volumes of solutions. The system contains aloading component, a detachable holding component, a combining componentand a receiving component.

Reactions can take place within plugs in any of these components.Reactions may also take place involving the surfaces of thesecomponents. The components may be placed into conditions (such astemperature, irradiation with light and other forms of radiation,exposure to various fields, etc) that maximize the desirable reactions.For example, if ligands and metal ions are loaded into the plugs of thecomponent, the formation of the ligand-metal complexes may proceedinside the plugs. The components may be placed under conditions tominimize the undesirable reactions. For example, when aqueous solutionsof biomolecules and/or organic molecules are in plugs, undesirablehydrolysis may take place. This hydrolysis may be minimized bycontrolling the temperature of the component, and also by freezing theplugs inside the component. Photobleaching of reagents inside the plugsinside the component may be minimized by minimizing the exposure of theplugs to light, either by using components that are substantially nottransparent to light, or by minimizing the exposure of the component tolight. If reactions of reagents inside the plugs with oxygen areundesirable, then the component may be fabricated with gas-impermeablematerials such as glass, or the component may be maintained underoxygen-free atmosphere. In addition, the carrier fluid (such as afluorinated fluid) may be degassed (for example, by freeze-pump-thawcycles or by bubbling with an inert gas such as argon) to remove anydissolved oxygen.

The types of reactions which can be conducted within plugs are notlimited. Examples include, but are not limited to, proteincrystallization, synthetic reactions, screening and enzymatic reactions,and diagnostic assays. Throughout this disclosure the use of the presentinvention is exemplified by application to protein crystallography. Itshould be understood that this invention is equally applicable to otherreactions.

Plug-based microfluidic manipulation of solutions has been describedpreviously by the present inventors in U.S. Ser. Nos. 10/434,970 and10/765,718, incorporated herein in their entirety by reference.

The present invention also provides a method for conducting reactions inplugs. The method comprises (a) introducing from a holding component alinear array of plugs of first plug fluid separated from each other by afirst carrier and/or spacer into a first microchannel within a combiningcomponent with a first flow rate; (b) introducing either (i) a stream ofsecond plug fluid or (ii) a stream of plugs of a second plug fluidseparated from each other by a second carrier into a second microchannelwithin the combining component with a second flow rate; and (c) mergingat least one plug of first plug fluid with at least some second plugfluid to form a merged plug, wherein said merging occurs at a junctionin fluid communication with the first and second microchannels.

In another embodiment, the linear array of reagent plugs is introducedinto the microchannel by attaching a holding component within which isdisposed a linear array of reagent plugs. Because the reagent plugs canbe stored in the holding component for long periods without loss ofintegrity, the array of plugs can contain active nucleation particles(such as aggregates of protein molecules or simply nanoparticles). Thereagent plugs can be stored and/or transported, and used later to inducenucleation for the crystallization or precipitation of proteins or othermolecules or materials. For example, the researcher could purchase aholding component containing an array of reagent plugs for a proteincrystallization screen, and then use a simple junction, and a simplesource of flow (such as a syringe or pressure) to merge the array ofmerged plugs with a stream of reagent or a stream of plugs.

For some applications, a plurality of different holding components maybe used together. For example, a capillary (a first holding component)containing 96 precipitants for protein crystallization in plugs may beprepared using a loading component. Separately, a second holdingcomponent (for example, another capillary) may be prepared with plugscontaining a protein to be crystallized. In this example, acrystallization experiment is carried out by merging the plugs of theprotein sample with the plugs of the precipitant (using techniquesdescribed elsewhere in this application, or in co-pending applicationSer. No. 10/434,970 or 10/765,718). For example, the first and secondholding components may be connected by a T-junction and their contentsforced into a third holding component.

The components containing the merged plugs can also be subjected to oneor more of light, an electric field, a magnetic field, heat, radiation,etc. to conduct reactions (including protein crystallization) andvarious studies on the merged plugs. For example, nucleation of proteincrystals may be induced in this way. For example, a component containingthe merged plugs can be subject to a temperature gradient to understandthe effect of temperature on the reaction. Ultimately, by tracking thereaction and by manipulating temperature, active control of the reactioncan be achieved.

Plugs can be sorted. For example, the plugs can also be sorted accordingto their sizes. Alternately, the plugs can be sorted by their densityrelative to that of the carrier fluid. Alternately, plugs can also besorted by applying a magnetic field if one group of the plugs containsmagnetic materials such as iron or cobalt nanoparticles or ferrofluid.

Reactions in plugs can be monitored in any of the components of themicrofluidic system or can be monitored after exiting the components ofthe microfluidic system. Plugs can be monitored using a variety oftechniques including fluorescence polarization, fluorescenceperturbation, fluorescence correlation spectroscopy, mass spectroscopy,etc. For example, crystals can be removed from the microfluidic systemand analyzed with x-ray diffraction. Plugs can be removed from themicrofluidic system and analyzed with mass spectroscopy.

Control of the Distance Between Plugs

The distance separating two plugs can be controlled with flow if thecarrier is chosen properly. In FIG. 5c , flow is applied to the channelsuch that the plugs remain separated. In FIG. 5d , no flow occursthrough the channel; if the carrier is chosen properly, the distancebetween plugs will decrease and ultimately the plugs will merge, ifallowed.

The distance of two plugs can be decreased in a number of ways. Forexample, a channel whose diameter increases along its length can beused. The distance between plugs would decrease if the plugs are movedtowards the widening of the channel. In some cases, merger can also bepromoted by increasing the diameter of the microchannel as the plugswill eventually merge as they move through such a channel. In somecases, obstacles, expansions or constrictions can be introduced tochange the velocity of the plugs and also to control merging.

Another way to decrease the distance of two plugs is to gradually removethe fluid between two plugs. For example, two plugs separated by a gasbubble can be moved closer together by applying pressure on either sideof the plugs. If the wall of the microchannel is permeable to gas, thegas can be purged out. The two plugs then get closer to one other andeventually merge. If the wall is not permeable, as in the case of aglass capillary, the gas is compressed and the volume decreased,therefore, the distance between the two plugs may be still decrease, butmerger would be prohibited.

FIG. 7 shows a means for changing the distance between plugs. When thepressure in the microchannel is higher than the pressure in the sidebranch channel, the carrier is removed and the distance between plugsdecreases (and eventually merges). When the pressure in the side branchchannel is higher than the pressure in the microchannel, then carrier isadded and the distance between plugs increases. Carrier can be replacedusing this design by having one side branch channel removing carrier,and another side branch channel adding another carrier.

If further manipulations are required (further reactions, treatment,merging and/or sorting, for example) the merged plugs could betransported to another holding component or a receiving component.

Crystallization of Biological Macromolecules

Microbatch and vapor diffusion are the two basic crystallization methodsfor the plug-based microfluidic system. Pending U.S. application Ser.No. 10/765,718 describes these techniques in a plug-based microfluidicssystem, and is herein incorporated by reference.

Using the present invention, plugs containing solutions forcrystallization can be loaded into a holding component using a loadingcomponent. After using a combining component to add a solution of themacromolecule of interest, crystallization trials can then be conductedin the receiving component when flow is stopped. The present inventionis capable of handling solutions having a wide variety of surfacetensions, viscosities, and wetting behaviors. Additionally, it allowsminimally invasive evaluation of crystal quality by diffraction while inthe receiving component, thus avoiding damage of fragile crystals byhandling. In addition, crystals can be removed from the receivingcomponent. The receiving component may be cut or otherwise disassembledto allow for the removal of crystals, or crystals may be flowed out ofthe intact receiving component. Removal of crystals may allow foroptional cryoprotection and freezing in order to obtain diffractiondata.

During the incubation in the receiving component, various fields(including, but not limited to, optical, magnetic, and electrical) maybe applied to induce nucleation and to control growth of the crystals.The environment of the holding component may also be controlled tooptimize the nucleation and growth of crystals. The receiving componentmay be designed to be substantially impermeable (for example, glass) tocomponents of plugs fluids, carrier fluids, and spacers, and also to theoutside elements. Alternatively, the receiving component may be designedto allow permeability of some of the components of plugs fluids, carrierfluids, and spacers, and/or the outside elements. Such permeability maybe used to control and optimize nucleation and growth of crystals. Forexample, selective removal or addition of water through the walls of theholding component (e.g. made of PDMS) may be used to control the rate ofnucleation and growth.

These techniques may be incorporated into, for example sparse matrix,gradient, or hybrid screening methods in the context of, for example,microbatch, vapor diffusion and free interface diffusion crystallizationmethods.

While this disclosure often refers to crystallization of amacromolecule, it should be understood that these methods are equallyapplicable to crystallization and co-crystallization of severalmacromolecules, and of their complexes.

The application of the invention described here to crystallization isillustrated using macromolecular crystallization as an example, but itis not limited to the crystallization of macromolecules.

Crystallization of Membrane Proteins

Structural characterization of membrane proteins is an important problemin human health, but structural determination is a challenging problem.The low availability of membrane protein solutions, and the large numberof conditions that need to be screened, have contributed to thisproblem, but are overcome by the present invention. Membrane proteinspose additional challenges due to their hydrophobic transmembraneportion, which typically requires the use of detergents for membraneextraction and crystallization. The presence of detergents can alter thecorrect formation of aqueous plugs formed in oil-based carrier fluidsinside hydrophobic microchannels, unless the surface tensions andwetting are controlled.

Suitable carriers can include those described above. Mixtures ofperfluoroamines, such as perfluoro-tri-n-butylamine andperfluoro-tri-n-dibutylmethylamine (also known as FC-40 and FC-70compounds) can be used as carrier fluids. Such mixtures have beendiscovered to be useful for formation of plugs when plugs are formedusing detergent solutions. This allows plugs containing membraneproteins solubilized in detergent solutions to be conveniently handledusing the present invention. Alternatively, other carrier fluids withsuitable wetting and surface tension properties may be used. Mixtures ofsuitable carrier fluids may be prepared in order to tailor propertiessuch as viscosity.

Approaches to Screening for Crystallization Conditions

Sparse matrix screening is often the first step used to find optimalcrystallization conditions. A sparse matrix screen is designed tosparsely sample the very large matrix of possible components andconcentrations of components that might comprise a solution thatfacilitates crystallization. The sparse matrix screening method may beimplemented by producing an array of plugs in a carrier fluid,optionally separated by spacers. Such plugs may contain a variety ofcrystallization conditions, including variations in the type andconcentration of precipitant, variation of pH and ionic strength, aswell as variation of the type and concentration of additives orcryosolvents. Such an array of plugs may be produced as describedelsewhere in this application. Preferably, such an array is produced ina holding component by a loading component. A disadvantage of the sparsematrix approach is that only a few concentrations are tested for eachreagent or a combination of reagents, and the best crystallizationconditions may be missed. A sparse matrix screen samples a large regionof chemical space, but it samples it with low density.

The gradient screen is an additional approach that overcomes thisdisadvantage. A gradient screen is designed to test a range ofcrystallization conditions such as the concentration of a solutioncomponent or the pH of the solution. These gradients are usuallydesigned to test a range of conditions that are known in the art to havea high probability of success, or to test a range of conditionssurrounding conditions that have shown indications of success usingother screening approaches. Gradient screening can be convenientlyimplemented by the present invention, as describe elsewhere in thisapplication. A disadvantage of the gradient screening approach is thatwhile it thoroughly explores one range of possible conditions, a largevariety of unrelated conditions go untested. A gradient screen samples asmaller region of chemical space, but it samples it with higher density.

A hybrid screening approach may combine the sparse matrix and gradientscreen approaches. This approach allows for both the broad screening ofpossible crystallization conditions with a more complete sampling ofconditions likely to prove successful. A hybrid screen may be performedusing a holding component that contains plugs with multiplecrystallization mixtures, where each mixture is present at severalconcentrations in several plugs. Generating a large array of plugs, asdescribed in the present invention, allows this hybrid screeningapproach to be used effectively, facilitating the rapid identificationof the ideal or near-ideal crystallization conditions with minimalsample consumption.

All screening approaches may be implemented redundantly, where severalsubstantially identical plugs are used for each crystallizationcondition. Typically, once an initial holding component is assembled,combined with a protein solution and allowed to incubate in a receivingcomponent, the plugs are observed for signs of crystallization. If, in afirst experiment using the present invention, or in a separateexperiment using other techniques known in the art, initial conditionsamenable to crystallization are determined (for example, by theobservation of the formation of crystals too small for structuredetermination or by observation of ordered precipitate, spherulites,etc.), a subsequent holding component may be designed to optimize thecrystal growth conditions. The solutions included in the subsequentholding component for optimization can include a range of conditionssurrounding conditions that have shown indications of success, as wellas a variety of solutions possessing similar properties as those seen tobe successful in the first array of plugs. For example, if a crystallineneedle is seen to grow in a plug containing 10% PEG 5000 during aninitial screen, a subsequent holding component designed to optimizecrystal growth may be designed to produce a concentration gradient ofPEG 5000 from about 1% to about 20%, as well as a gradient of otherPEGs, such as PEG 2000, PEG 8000, or PEG-MME 5000. Optionally, a seriesof other crystallization additives not included in the first array ofplugs can be included in the subsequent array of plugs. The subsequentarray of plugs can be predesigned to be used with proteins havingcertain defined results in experiment using the first array of plugs, orcan be custom designed based upon the results of a specificmacromolecule in experiments using the first array.

In order to facilitate such experiments, kits could be assembled thatcontain holding components with sparse matrix screen reagents along withholding components that contain fine screens for selected conditionspresent in the sparse matrix screen holding component. In certainalternative embodiments, a holding component could contain a largenumber of fine screens. This approach, however, would consume moreprotein than sequentially using a sparse screen holding componentfollowed by one or more selected fine screen holding components.

Computer software may be provided as parts of kits. This software mayhelp analyze the result of the first set of experiments, and then helpthe user design the next set of experiments. Optionally, this softwaremay contain a database of recipes, and may contain rules for using theresults of one set of experiments to design the next set of experiments.Such software may also be used to process the available information ofthe macromolecule to be crystallized (such as the protein sequence andthe method of synthesis and purification), and propose an initial set ofcrystallization conditions. The software may use this information alsoduring the design of the next set of experiments.

Microbatch Crystallization

To perform microbatch screening, a combining component is used to mergea solution of a macromolecule with an array of plugs in a holdingcomponent containing crystallization reagents. The plugs are thentransported into a receiving component for incubation and crystalgrowth. The solution of a macromolecule used can be an array of plugs,or a continuous stream of macromolecule solution. Plugs of solution of amacromolecule can be generated as described elsewhere in thisapplication.

Vapor Diffusion Crystallization

The present invention can be used to perform Vapor Diffusion (VD)Crystallization. In this approach to VD crystallography, it is helpfulto think of pairs of plugs in an alternating series, where the plugs arein a carrier fluid optionally containing spacers between members of sucha pair and/or between groupings of plugs. In one embodiment, a first setof plugs contain a macromolecule and precipitant solution, and alternatewith a second set of “empty” plugs (that is, substantially free ofmacromolecule) containing a desiccant solution. For example, thedesiccant solution may contain the precipitant from the macromoleculeand precipitant solution at a higher concentration than in themacromolecule and precipitant solution. During incubation, when thecarrier fluid is substantially water-permeable or the plugs areseparated by a permeable spacer such as a gas bubble, water moleculeswill diffuse over time from the first set of plugs with low osmoticpressure to the second “empty” set of dessicant plugs with high osmoticpressure. For example, water will diffuse from a first plug containing alow concentration of macromolecule and precipitant to a second plugcontaining a high concentration of salt solution, thus increasing theconcentration of macromolecule and precipitant in the first plug. Thisincrease in concentration can cause the crystallization of themacromolecule. Crystallization by this technique is not limited,however, to the scenario in which the concentration of the plugcontaining macromolecule and precipitant increases in concentration.Crystallization can also occur by a decrease in concentration in theplug containing macromolecule, as, for example, when a macromoleculecrystallizes due to lowered ionic strength of its solution. This changecould be accomplished in an alternative embodiment in which the second“empty” set of plugs has a lower ionic strength than the first set ofplugs containing the macromolecule, so water molecules diffuse into thefirst set of plugs, lowering the ionic strength.

By varying the concentrations of the plug fluids, varied solutions ofmacromolecule and precipitant can be paired with “empty” plugscontaining solutions at a variety of osmotic pressures, allowing theestablishment of multiple crystallization experiments within a singledevice. The rate and the extent of the transfer of water can becontrolled by varying the difference in osmotic pressure and the spacingbetween the plugs in each pair, and by the relative size of the “empty”plugs and the plugs containing macromolecule. Note that the plug fluidin macromolecular crystallization commonly contains water, but it is notlimited to water. The solution in the “empty” plugs preferably has anosmotic pressure different from that of the macromolecule andprecipitant solution. The “empty” plugs may also contain a differentconcentration of a component that may be transferred between the “empty”plugs and the crystallization plug.

To create an alternating set of “empty” plugs, in the combiningcomponent a stream of macromolecule solution will selectively merge withprecipitant plugs but preferably not with the second set of plugs. Asdescribed above, the selective merging of precipitant plugs with astream of macromolecules is possible through the manipulation ofrelative surface tensions and/or viscosities inside the “empty” plugsand the precipitant plugs, and based on the geometry of the combiningcomponent. This system may provide reliable merging with nocross-contamination, as described elsewhere in this application.Preparation of pairs of precipitant plugs and desiccants plugs can begenerated by a loading component, controlling the spacing of the plugsas well as the presence or absence of spacers, and loaded into in aholding component as described elsewhere in this application. A solutionof macromolecule can then be selectively merged with the precipitantcomponent using a combining component and then incubated in a receivingcomponent.

Note that while this description of vapor diffusion crystallographyincludes one set of plugs with macromolecule and a second “empty” set ofplugs, the method would be operational even if the second set of plugswere not “empty” but instead also contained macromolecule. However theselective merging techniques described here allow the use of smallerquantities of protein.

Free Interface Diffusion Crystallization

Free interface diffusion has the potential to generate well-orderedcrystals. The present invention can also be used to perform FreeInterface Diffusion (FID) crystallization. To crystallize macromoleculesby FID methods, a reservoir of macromolecule solution must be broughtinto contact with a reservoir of precipitant solution so that afree-standing interface is formed. The reagents mix preferably by simplediffusion and preferably convection is minimized. The convection may beminimized by minimizing the Grashoff number, the ratio of convective tobuoyant forces, at the interface. In addition, convection may beminimized by reducing surface tension gradients at the liquid-liquid andliquid-solid interfaces, and increasing viscosity to suppress convectiveflows.

The present invention comprises a microfluidic method to form afree-interface between two plugs. The two plugs are brought together ina microchannel device, preferably in a receiving component, such thatthey are partially separated by an immiscible carrier and/or spacer. Theplugs are allowed to come into contact at a point where there is noimmiscible fluid present, thereby establishing a free-standing interfacefor fluids and reagents to diffuse between the plugs. In someembodiments, the contact between plugs occurs in a hydrophilicmicrochannel where the plug fluids wet the wall, bringing the contentsof the two plugs into contact and allowing the reagents to mix through athin fluid interface. A spacer or a volume of the carrier fluid may beused to constrain this interface. In other embodiments, plug fluids maywet a thin filament, establishing an interface.

Methods for controlling the spacing of plugs and the control of wettingof the microchannel by the plug fluids are discussed below.

Plug Manipulation in VD and FID Crystallography

In both FID and VD, the spacing between plugs and the wetting phenomenacan be controlled simultaneously by depositing plugs in a separatechannel from the channel they are formed in. In one embodiment, first,any sequence of plugs with arbitrary spacing is formed in the firstmicrofluidic device where the plugs do not wet the channel walls. Theoutlet of the first device is placed into a second device with an innerdiameter large enough to contain the first device. The first and secondmicrofluidic devices are translated relative to one another (forexample, the first is withdrawn from the second) while plugs aretransported out the outlet of the first, and the plugs are subsequentlydeposited into the second microfluidic device. The surfaces of thesecond microfluidic device are treated such that the plug fluids areallowed to wet the second device, and to form an interface. The rate atwhich plugs are deposited and the rate at which the two devices aremoved relative to one another determines the spacing between plugs andallow the user to arbitrarily control this spacing.

Additionally, the second microfluidic device may be modified so thatwetting is more precisely controlled. Hydrophilic and hydrophobicpatches (or other patches that allow variability of the surface energyand can control wetting properties of plug fluids and carrier fluids)may be patterned directly on the inside channel walls so that plugs onlywet in limited regions. These patches may be masked, and may beactivated upon an external or internal trigger, for example atemperature change or irradiation with light or presence of a particularcomponent in the plug fluid or carrier fluid. Also, a filament, wire, orother surface may be inserted into the microfluidic device with similarpatterning to direct wetting only along the length of the filament orwire instead of the walls. The second microfluidic device, ifconstructed out of glass or another fluid-impermeable material, preventsevaporation of the carrier and reagent fluids.

If the surface of the microchannel is prepared so that wetting is notallowed, the plugs will maintain discrete distances (see FIG. 5a ). Ifwetting by plug fluids is allowed, the plug fluids will come in contact(see FIG. 5b ). The surface of the microchannel can have differentwetting characteristics. For example, a portion of the microchannel canbe hydrophobic. When aqueous plugs are used, this hydrophobic surfacewould prevent wetting. If another portion of the microchannel ishydrophilic (either on its surface or due to the presence of ahydrophobic fiber), the plug fluids of the alternating plugs willinterdiffuse because the hydrophilic surface promotes wetting.Alternatively, wetting of the inner surface of the microfluidics devicecan be controlled by applying a photoswitchable monolayer. Initially themonolayer would prevent wetting (FIG. 5e ), but when activated withlight would promote wetting (FIG. 5f ).

Several additional examples of methods for bringing two plugs togetherto form a free-standing interface are shown in FIG. 5. Plugs of reagentsolutions are initially formed in a channel under conditions thatprevent them from wetting the walls of the channel (FIG. 5a ). Theconditions are then changed so that the plugs wet the walls (FIG. 5b ).If the plugs are sufficiently close, then the wetting phenomena allowsthe plug fluids to come into contact with each other. For example,aqueous plugs may be transported into a hydrophilic channel but a thinlayer of carrier fluid prevents the plugs from wetting while flow ismaintained (FIG. 5c ). When the flow stops, the thin layer of carrierfluid disperses, allowing the plug fluids to wet the wall (FIG. 5d ). Asanother example, the microchannel walls may be treated with aphotoswitchable monolayer that expresses hydrophobic propertiesinitially (FIG. 5e ) but then expresses hydrophilic properties uponexposure to a particular wavelength of light (FIG. 5f ).

In a further example, the microfluidic system can be formed, in whole orpart, from materials that are thermo-responsive. Composite materialswith poly(N-isopropylacrylamide) coated on rough silicon substrate arehydrophilic (contact angle of water of ˜0°) at low temperatures andhydrophobic (contact angle of water of ˜150°) at high temperatures. Thehydrophilicity/hydrophobicity transition temperature is between 29°C.˜40° C., and the process proceeds within several minutes.Macromolecules may denature at these temperatures, but it is possible toheat the hydrophobic microchannel with the thermoresponsive fiber toabove 40° C., quickly cool it to 25° C. (or 18° C. or 4° C.) and injectthe macromolecule and precipitant plugs into the microchannel before thetransition occurs. After several minutes, the fiber will becomehydrophilic and a liquid bridge will form between the macromolecule andprecipitant plugs allowing diffusion to occur.

X-Ray Crystallography

The quality of the crystals grown in plugs in a receiving component canbe evaluated directly by diffraction at room temperature or otherdesired temperature (for example, the temperature at which the crystalswere grown) in their original mother liquor (FIG. 8). In someembodiments, structural information may be obtained, either from onecrystal, or from several crystals either in the same plug or indifferent plugs. Alternatively, crystals that form in the plugs insidethe capillary can be easily extracted, cryoprotected, frozen, anddiffracted. Since crystals may be grown at a liquid/liquid interfacethey would not need to be scraped off a solid surface, minimizingdamage.

FIG. 3c illustrates a microfluidics device 1 in which variousprecipitant plugs 7 separated by a carrier 6 are merged withmacromolecule plugs 13 separated by a carrier. The precipitant plugstypically have varying concentrations of precipitating agent. The mergedplugs 10 then flow through a downstream channel where they can befurther manipulated, monitored, collected, etc. The carrier 6 used toseparate the precipitant plugs and the macromolecule plugs can be thesame or different. FIG. 3c illustrates an example of merging of theprotein stream with a series of plugs.

Method of Providing an Array of Plugs

The present invention also provides a method of providing an array ofplugs in a holding component to a customer, comprising the steps ofoffering an inventory of reagents, carrier and plug fluids; receivingfrom the customer a desired subset of reagents, a desired carrier and adesired plug fluid; forming an array of plugs in a holding component,where plugs are separated from each other with a carrier and where eachplug contains an element of the desired reagent subset, and deliveringthe holding component to the customer. The method can additionallycomprise calculating a price for the subset of desired reagents.

The method may involve pre-fabricated arrays of plugs of desiredcomposition, storing these arrays, and providing them to the customer.

The customer may also be provided with a kit comprising a loadingcomponent, and a series of holding components, combining components, andreceiving components. The components can have integral matching fluidicconnections, or separate connectors can be used to link them, or both.The customer may utilize these kits to generate arrays of plugs ofdesired composition. Reagents both proprietary to the customer andnon-proprietary may be used.

The inventory of reagents, carriers and plug fluids can be offered as acatalog either in paper or electronic form (in some embodimentsaccessible via the internet). Suitable reagents, carriers and plugsfluids are described throughout the present disclosure. For proteincrystallography, the reagents can include any compound used to screenfor crystal formation. For example, reagents can include precipitationagents, additives, cryosolvents, buffering agents, etc.

For example, a list of companies and products relating to reagents andkits for protein crystallography is described athttp://www.structmed.cimr.cam.ac.uk/Course/Crystals/screening.html.

In this method, the customer selects various features of the array ofplugs within the holding device: such as one or more precipitationagents, the carrier, the plug fluid, the pH of the plug fluid,additives, cryosolvents, etc.

For membrane proteins and other macromolecules, the plug fluid can be alipidic cubic phase (see e.g. Landau, E. M. & Rosenbusch, J. P. Lipidiccubic phases: A novel concept for the crystallization of membraneproteins. Proceedings of the National Academy of Sciences of the UnitedStates of America 93, 14532-14535 (1996), and Nollert, Methods.34(3):348-53 (November 2004)).

A nonexclusive list of salts that may be used as precipitation agents isas follows: tartrates (Li, Na, K, Na/K, NH₄); phosphates (Li, Na, K,Na/K, NH₄); acetates (Li, Na, K, Na/K, Mg, Ca, Zn, NH₄); formates (Li,Na, K, Na/K, Mg, NH₄); citrates (Li, Na, K, Na/K, NH₄); chlorides (Li,Na, K, Na/K, Mg, Ca, Zn, Mn, Cs, Rb, NH₄); sulfates (Li, Na, K, Na/K,NH₄); maleates (Li, Na, K, Na/K, NH₄); glutamates (Li, Na, K, Na/K,NH₄); tetraarylborates (Li, Na, K, Na/K, NH₄).

A nonexclusive list of organic materials that may be used asprecipitation agents is as follows: PEG 400; PEG 1000; PEG 1500; PEG 2K;PEG 3350; PEG 4K; PEG 6K; PEG 8K; PEG 10K; PEG 20K; PEG-MME 550; PEG-MME750; PEG-MME 2K; PEGMME 5K; PEG-DME 2K; dioxane; methanol; ethanol;2-butanol; n-butanol; t-butanol; jeffamine m-600; isopropanol;2-methyl-2,4-pentanediol; 1,6-hexanediol.

Solution pH can be varied by the inclusion of buffering agents; typicalpH ranges for biological materials lie anywhere between values of 3 and10.5 and the concentration of buffer generally lies between 0.01 and0.25 M. The microfluidic devices described in this document are readilycompatible with a broad range of pH values, particularly those suited tobiological targets.

A nonexclusive list of possible buffers that may be used according tothe invention is as follows: Na-acetate; HEPES; Na-cacodylate;Na-citrate; Na-succinate; Na-K-phosphate; TRIS; TRIS-maleate;imidazole-maleate; bistrispropane; CAPSO, CHAPS, MES, and imidazole.

Additives are small molecules that affect the solubility and/or activitybehavior of the target. Such compounds can speed up crystallizationscreening or produce alternate crystal forms or polymorphs of thetarget. Additives can take nearly any conceivable form of chemical, butare typically mono and polyvalent salts (inorganic or organic), enzymeligands (substrates, products, allosteric effectors), chemicalcrosslinking agents, detergents and/or lipids, heavy metals,organometallic compounds, trace amounts of precipitating agents, andsmall molecular weight organics.

The following is a nonexclusive list of additives that may be used inaccordance with the invention: 2-butanol; DMSO; hexanediol; ethanol;methanol; isopropanol; sodium fluoride; potassium fluoride; ammoniumfluoride; lithium chloride anhydrous; magnesium chloride hexahydrate;sodium chloride; calcium chloride dihydrate; potassium chloride;ammonium chloride; sodium iodide; potassium iodide; ammonium iodide;sodium thiocyanate; potassium thiocyanate; lithium nitrate; magnesiumnitrate hexahydrate; sodium nitrate; potassium nitrate; ammoniumnitrate; magnesium formate; sodium formate; potassium formate; ammoniumformate; lithium acetate dihydrate; magnesium acetate tetrahydrate; zincacetate dihydrate; sodium acetate trihydrate; calcium acetate hydrate;potassium acetate; ammonium acetate; lithium sulfate monohydrate;magnesium sulfate heptahydrate; sodium sulfate decahydrate; potassiumsulfate; ammonium sulfate; di-sodium tartrate dihydrate; potassiumsodium tartrate tetrahydrate; di-ammonium tartrate; sodium dihydrogenphosphate monohydrate; di-sodium hydrogen phosphate dihydrate; potassiumdihydrogen phosphate; di-potassium hydrogen phosphate; ammoniumdihydrogen phosphate; di-ammonium hydrogen phosphate; tri-lithiumcitrate tetrahydrate; tri-sodium citrate dihydrate; tri-potassiumcitrate monohydrate; diammonium hydrogen citrate; barium chloride;cadmium chloride dihydrate; cobaltous chloride dihydrate; cupricchloride dihydrate; strontium chloride hexahydrate; yttrium chloridehexahydrate; ethylene glycol; Glycerol anhydrous; 1,6 hexanediol; MPD;polyethylene glycol 400; trimethylamine HCl; guanidine HCl; urea;1,2,3-heptanetriol; benzamidine HCl; dioxane; ethanol; iso-propanol;methanol; sodium iodide; L-cysteine; EDTA sodium salt; NAD; ATP disodiumsalt; D(+)-glucose monohydrate; D(+)-sucrose; xylitol; spermidine;spermine tetra-HCl; 6-aminocaproic acid; 1,5-diaminopentane diHCl;1,6-diaminohexane; 1,8-diaminooctane; glycine; glycyl-glycyl-glycine;hexaminecobalt trichloride; taurine; betaine monohydrate;polyvinylpyrrolidone K15; non-detergent sulfo-betaine 195; non-detergentsulfo-betaine 201; phenol; DMSO; dextran sulfate sodium salt; JeffamineM-600; 2,5 Hexanediol; (+/−)-1,3 butanediol; polypropylene glycol P400;1,4 butanediol; tert-butanol; 1,3 propanediol; acetonitrile; gammabutyrolactone; propanol; ethyl acetate; acetone; dichloromethane;n-butanol; 2,2,2 trifluoroethanol; DTT; TCEP; nonaethylene glycolmonododecyl ether, nonaethylene glycol monolauryl ether; polyoxyethylene(9) ether; octaethylene glycol monododecyl ether, octaethylene glycolmonolauryl ether; polyoxyethylene (8) lauryl ether;Dodecyl-β-D-maltopyranoside; Lauric acid sucrose ester;Cyclohexyl-pentyl-β-D-maltoside; Nonaethylene glycol octylphenol ether;Cetyltrimethylammonium bromide;N,N-bis(3-D-gluconamidopropyl)-deoxycholamine;Decyl-β-D-maltopyranoside; Lauryldimethylamine oxide;Cyclohexyl-pentyl-β-D-maltoside; n-Dodecylsulfobetaine,3-(Dodecyldimethylanimonio)propane-1-sulfonate;Nonyl-β-D-glucopyranoside; Octyl-β-D-thioglucopyranoside, OSG;N,N-Dimethyldecylamine-β-oxide; Methyl0-(N-heptylcarbamoyl)-α-D-glucopyranoside; Sucrose monocaproylate;n-Octanoyl-β-D-fructofuranosyl-α-D-glucopyranoside;Heptyl-β-D-thioglucopyranoside; Octyl-β-D-glucopyranoside, OG;Cyclohexyl-propyl-β-D-maltoside;Cyclohexylbutanoyl-N-hydroxyethylglucamide; n-decylsulfobetaine,3-(Decyldimethylammonio)propane-lsulfonate; Octanoyl-N-methylglucamide,OMEGA; Hexyl-β-D-glucopyranoside; Brij 35; Brij 58; Triton X-114; TritonX-305; Triton X-405; Tween 20; Tween 80; polyoxyethylene(6)decyl ether;polyoxyethylene(9)decyl ether; polyoxyethylene(10)dodecyl ether;polyoxyethylene(8)tridecyl ether; Decanoyl-N-hydroxyethylglucamide;Pentaethylene glycol monooctyl ether;3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate;3-[(3-Cholamidopropyl)-dimethylammonio] hydroxy-1-propane sulfonate;Cyclohexylpentanoyl-N-hydroxyethylglucamide;Nonanoyl-N-hydroxyethyglucamide;Cyclohexylpropanol-N-hydroxyethylglucamide;Octanoyl-N-hydroxyethylglucamide;Cyclohexylethanoyl-N-hydroxyethylglucamide; Benzyldimethyldodecylammonium bromide; n-Hexadecyl-β-D-maltopyranoside;n-Tetradecyl-β-D-maltopyranoside; n-Tridecyl-β-D-maltopyranoside;Dodecylpoly(ethyleneglycoether); n-Tetradecyl-N,N-dimethylammonio-1-propanesulfonate; n-Undecyl-β-D-maltopyranoside; n-DecylD-thiomaltopyranoside; n-dodecylphosphocholine; α-D-glucopyranoside,β-D-fructofuranosyl monodecanoate, sucrose mono-caprate;1-s-Nonyl-β-D-thioglucopyranoside; n-Nonyl-β-D-thiomaltoyranoside;N-Dodecyl-N,N-(dimethlammonio)butyrate; n-Nonyl-β-D-maltopyranoside;Cyclohexyl-butyl D-maltoside; n-Octyl-β-D-thiomaltopyranoside;n-Decylphosphocholine; n-Nonylphosphocholine;Nonanoyl-N-methylglucamide; 1-s-Heptyl-β-D-thioglucopyranoside;n-Octylphosphocholine; Cyclohexyl-ethyl D-maltoside;n-Octyl-N,N-dimethyl ammonio-1-propanesulfonate;Cyclohexyl-methyl-β-D-maltoside.

Cryosolvents are agents that stabilize a target crystal to flash-coolingin a cryogen such as liquid nitrogen, liquid propane, liquid ethane, orgaseous nitrogen or helium (all at approximately 100-120° K) such that acrystal becomes embedded in a vitreous glass rather than ice. Any numberof salts or small molecular weight organic compounds can be used as acryoprotectant, and typical ones include but are not limited to: MPD,PEG-400 (as well as both PEG derivatives and higher molecular-weight PEGcompounds), glycerol, sugars (xylitol, sorbitol, erythritol, sucrose,glucose, etc.), ethylene glycol, alcohols (both short- and long chain,both volatile and nonvolatile), LiOAc, LiCl, LiCHO₂, LiNO₃, Li₂SO₄,Mg(OAc)₂, NaCl, NaCHO₂, NaNO₃, etc. Again, materials from whichmicrofluidics devices in accordance with the present invention arefabricated may be compatible with a range of such compounds.

In addition to chemical variability, a host of other parameters can bevaried during crystallization screening. Such parameters include but arenot limited to: (1) volume of crystallization trial; (2) ratio of targetsolution to crystallization solution; (3) target concentration; (4)cocrystallization of the target with a secondary small or macromolecule;(5) hydration; (6) contact surfaces; (7) modifications to targetmolecules; etc.

For application (4), cocrystallization, the array may represent a subsetof small molecules that span a portion of chemical space. Alternatively,the array may represent a subset of oligonucleotides spanning a genomeor a portion of a genome.

Other Applications

The present invention may be used for any experiments in which a largevariety of reactions are to be performed in parallel, for example, anyapplications which are currently typically applied in 96-, 384- or1536-well plates. Holding components as described in the presentinvention may be prepared for any of these applications.

One such application is combinatorial chemistry. A holding component maybe prepared that contain a diversity of reagents to be reacted with asecond reagent or set of reagents after combination in a combiningcomponent. These reagents are not limited to being in solution, and maybe also presented on beads or in other forms. Typical applications ofcombinatorial chemistry include the synthesis of a diversity ofcompounds to be tested for pharmaceutical or agricultural activity.Another application is the synthesis of a diversity of inorganic,organic, or organometallic catalysts. For example, the holding componentmay contain a number of different potential ligands for a metal, andafter combination with a metal or variety of metals in the combiningcomponent, the ligands are allowed to react with the metal or metals toprovide an array of plugs containing the potential catalysts.Subsequently the plugs may be combined with a test reagent in a secondcombining component to allow the catalysts to be tested for catalyticactivity. Combinatorial chemistry reagents in a holding component willbe chosen such that, after being combined with a second reagent or sentof reagents and allowed to react, the final products will span a desiredregion of chemical space. In addition, the plugs in the holdingcomponent may contain reagents in a suspension. This may be useful forincreased stability, or when solubility is limited. The holdingcomponent may also be frozen for increased stability.

Another application is the screening of a diverse library of compoundsfor a desired property, frequently referred to as “high-throughputscreening”. For example, if a catalyst is needed to carry out a specificreaction to convert substance A to substance B, a number of potentialcatalysts can be loaded into a holding component, combined withsubstance A, allowed to react, and then the resulting plugs in areceiving component can be monitored for the presence of substance B. Inthis example, the catalyst could be inorganic, organometallic, organic,enzymatic or comprised of nucleic acids. Alternatively, a holdingcomponent could contain a diverse set of reactants in order to test theactivity of a potential catalyst. In a combining component, the reactantset would be combined with a potential catalyst and allowed to react ina receiving component. The course of the reactions would then bemonitored, for example by UV-vis spectroscopy, fluorescence, ormass-spectroscopy, to determine the reactivity of the catalyst.

Similarly, diverse sets of compounds or “libraries” can be loaded in aholding component to be tested for a desired biological activity, suchas for use as a pharmaceutical or agricultural compound. Examples oftypical libraries include a set of over 10,000 plant and microbialextracts offered by Sigma-Aldrich and PhytoMyco Research Corporation,the Library of Pharmacologically-Active Compound (Product No. LO1280)offered by Sigma-Aldrich, and compounds from the Aldrich Library of RareChemicals. Other examples include the Pharma Library Collection offeredby Nanosyn, Inc., that includes compounds preselected for theirpotential to be drugs based on their known properties and the ExploreLibrary Collection, also from Nanosyn, Inc., that contains more reactivecompounds. Other examples include Double Diversity arrays by NikemResearch, which contain a single reactive functional group to allowattachment of a customer's proprietary compounds, and Nikem's coMergearrays.

Other uses and examples of compound libraries are described in U.S. Pat.No. 6,740,506, incorporated herein by reference. In addition, a holdingcomponent can comprise libraries synthesized via directed evolution suchas those described in U.S. Pat. No. 6,740,506.

Similarly, the holding component can contain a diversity ofbiomolecules. For example, as described below in Example 10 a holdingcomponent could comprise a diverse set of proteins for testing forbinding to, or reacting with, a single substrate or a set of substrates.An example of a protein library is described in Tsuji et al., NucleicAcids Research 29(20):e97 (2001). The holding component could alsocontain a library of proteins expressed by a genome. Similarly, aholding component could contain oligopeptides or oligo- orpoly-saccharides. Alternatively, a holding component could containnucleic acids, such as DNA or RNA.

In one set of embodiments, a holding component could contain a set ofbiomolecules derived from a complete genome of any organism, includinganimals, fungi, Archaea, bacteria, plants, mammals, humans, etc., thathas been sequenced by means known in the art. A holding component couldcontain biomolecules representing the entire genome of an organism, or asubset of the genome. For example, the holding component could containnucleic acids representing a genome of an organism. The nucleic acidscould be RNA, DNA, cDNA or EST clones. One example is the Ez-rays™ humanOligonucleotide Library sold by Matrix Technologies which contains over15,000 50-mer DNA probes targeted to human mRNA sequences plus controlprobes.

In another set of embodiments, the holding component could contain livecells. For example, useful sets of cells include a deletion librarycontaining a large set of deletion mutants of an organism, such as thatproduced by the Saccharomyces Genome Deletion Project (Giaever et al.,Nature. 418(6896):387-91 (2002)). Other useful cell collections includesets of EST clones, BAC clones, ORF clones and PAC clones, for exampleas offered by Invitrogen Corporation. In other embodiments, a singletype of cell could be offered in a holding component for combining witha plurality of reagents. For example, hepatic cells, such as thosedescribed in O'Connor, et al., Cytometry A. 63(1):48-58 (2005), could beloaded in a holding component, and then combined in a combiningcomponent with a diversity of potential pharmaceuticals. Subsequentmonitoring of cell activity or viability could be used to test for livertoxicity. Alternatively, a variety of cell lines representingsubpopulations, also described in O'Connor, et al., Cytometry A.63(1):48-58 (2005), could be loaded into a holding component.

For either high-throughput screening or combinatorial chemistryapplications, an entire library of thousands of compounds could beoffered in a single holding component. Alternatively, as described abovefor protein crystallography, a sparse subset of the entire library couldbe offered in a single holding component, and then finer subsets of theentire library could be offered for use after a particular region of achemical space has been found to have desirable properties.

In another set of embodiments, the holding component could be used fordiagnostics and detection. Non-limiting example include medicaldiagnostics, veterinary diagnostics, the testing of crops, animals, andfarm products, and environmental testing including the testing of waterand air. Preferably, the holding component contains a plurality oftests, and/or multiple copies of the same test. The holding componentcan also contain control experiments.

In some embodiments of the invention, after an initial test using aspecific holding component containing a sparse library of substances, asecond custom holding component could be prepared containing substancesclose in properties (that is close in chemical space) to “hits” found inthe initial test. This process could be iterated indefinitely.

EXAMPLES

In the examples below, glass capillaries were silanized to render themhydrophobic.

Example 1 No-Loss Injection of Sub-μL Volumes of Solutions intoMicrofluidic Devices

A 10 μL syringe and a piece of Teflon tubing were filled with theperfluoroamine, followed by aspiration of a small volume (less than 1μL) of a membrane protein solution. Because the perfluoroamine carrierfluid preferentially wet the Teflon, it formed a thin layer between theprotein and Teflon, preventing sticking of the protein solution to thewalls of the tubing and allowed for complete, no-loss dispensing into amicrofluidic device.

Example 2 Forming an Array of Plugs of Different Composition in aHolding Component

In this experiment, array of plugs containing different composition weregenerated in Teflon tubing or in glass capillaries. A piece of Teflontubing (OD: 760 μm, ID: 305 μm) was attached to the needle of a 5 μLsyringe (Hamilton). Another piece of Teflon tubing with smaller diameter(OD: 250 μm, ID: 200 μm) was attached to the large Teflon tubing andsealed by either epoxy or wax. Oil (FC-3283/PFO at 50:1) was aspiratedinto the tubing and the syringe. The tubing was immersed into an aqueoussolution of composition A (solution A), and the plunger was pulled backto aspirate a small amount (0.1-0.5 μL) of solution A. Next the tubingwas immersed into the oil and the same amount of oil was aspirated intothe tubing. This two-step aspiration process was repeated for solutionB, C, . . . until all the solutions were used, and an array of plugs ofcomposition of A, B, . . . was formed inside the Teflon tubing.

To form such an array of plugs in a glass capillary, the Teflon tubingwas inserted into the wide end of a Hampton Research glass capillary (OD200 μm, ID 180 μm, with an approximately 2 mm-wide opening in a shape ofa funnel on one end). After the whole system (the syringe, the Teflontubing and the capillary) was filled with oil, equal amount of solutionA, oil, solution B, oil, . . . were aspirated into the capillaryfollowing the same procedure as described above.

When the array of plugs was formed inside the Teflon tubing, the Teflontubing was cut off and frozen with the liquid inside. The freezingprocess prevented liquid loss by permeation though Teflon, thus help thelong-term storage of the plugs. When the array of plugs was formedinside the glass capillary, the capillary was removed from the Teflontubing and sealed with wax. In this way plugs could be stored for a longtime (over 6 months).

Example 3 Transporting the Array of the Plugs into a PDMS Microchannel

In this experiment, the array of the plugs stored inside the Teflontubing or the glass capillary (holding component) was transported into aPDMS microchannel (receiving component).

Three different configurations were used. In the first configuration,the inlet of the PDMS microchannel was coupled to a glass capillaryobtained from Hampton research. This capillary was used as an adapterand it had a shape of a funnel. The small end of the capillary wasinserted into the PDMS microchannel and the gap between the capillaryand the wall of the PDMS microchannel was filled with half-cured PDMS.After incubation at 110° C. for 5 min, the half-cured PDMS wassolidified and the junction became leak-proof. The capillary that hadthe array of the plugs inside was firmly inserted into the larger end ofthe adapter capillary. The connection was leak-proof without anysealing. By tilting the whole setup, the gravity force would pull thearray of the plugs into the PDMS microchannel. Alternatively, thetransportation of the plugs could be achieved by using a syringe toapply a pressure on the open end of the capillary containing the array.

In the second configuration, the inlet of the PDMS microchannel wascoupled to a small Teflon tubing (OD: 250 μm, ID: 200 μm). The junctionwas made leak-proof in the same way as described in the firstconfiguration (using half-cured PDMS). The free end of the Teflon tubingwas then inserted into the larger end of the glass capillary that heldthe array of the plugs. Again, either the gravity force or a pressure onthe small end of the glass capillary applied by a syringe was applied totransfer the array of the plugs into the PDMS microchannel.

In the third configuration, the array of the plugs was first transportedinto a piece of small Teflon tubing that was connected to a syringe inthe same setup as above. Then the small Teflon tubing with the array ofthe plugs inside was inserted into the capillary coupler. By applyingpressure through the syringe, the array of the plugs can be pushed intothe PDMS microchannel.

The flow of the plugs may be controlled by the syringe that pushed theplugs into the microchannel. Alternatively, after the array of the plugsentered the PDMS microchannel and passed the oil inlet, the inlet may besealed by wax. Then the movement of the plugs can controlled by the flowof the oil, which was pushed by a syringe and a syringe pump.

Example 4 Transporting the Plugs from a PDMS Channel (ReceivingComponent) into a Glass Capillary (Holding Component)

In this experiment, the array of the plugs that has been merged withprotein solution individually was transported into a glass capillarythat was connected to the outlet of the PDMS microchannel. A reusableconfiguration could be employed here to reuse the PDMS microfluidicdevice. This was achieved by attaching a thin-walled (OD: 250 μm, ID:200 μm) Teflon tubing to the outlet of the PDMS microchannel. The glasscapillary (OD: 200 μm, ID: 180 μm) from Hampton research can be coupledto the Teflon tubing by inserting the free end of the tubing to thelarger end of the glass capillary. No sealing is needed for the couplingand no leaking was observed. After one glass capillary was filled withone array of the plugs that contain the mixture of the protein solutionand the various precipitants, it was pulled off from the Teflon tubingand sealed by wax. Another glass capillary can be used to couple to theTeflon tubing for the next array of the plugs.

Example 5 Merging Each Plug in the Array with a Protein Solution in aPDMS Microchannel

In this experiment, there are two configurations to carry out themerging. In the first configuration, the array of plugs was pushed in amerging component, +past a junction with a channel into which was floweda stream of the protein solution. Each plug would merge with a smallamount of protein solution. By varying the flow rate of the plugs or theflow rate of the protein stream, the relative volume ratio of eachprecipitant to protein can be controlled.

In the second configuration, the protein stream was first injected intoa flow of oil so that another array of plugs of protein solution wasformed. The array of the plugs containing the precipitants can bebrought to merge with the array of the plugs of protein solution one byone at a T-junction in a merging component.

Example 6 Merging Every Other Plug with Protein Solution in a PDMSMicrochannel

An array of the plugs may be composed of many groups of plugs. It issometimes desirable for the combining component to merge a solutionselectively amongst these groups. For example, in the case of VDcrystallography, a first group of plugs would contain a precipitantsolution, while a second group of plugs would contain a desiccantsolution. To perform crystallization by vapor diffusion, it is desirablefor a stream of the protein solution to merge with precipitant plugs butnot desiccant plugs. The merging of an array of plugs with a reagentsolution can be described by the Capillary number, which is dependent onsurface tension and viscosity. In the preferred embodiment, the surfacetensions between the plugs and the carrier fluid can be controlled,allowing for control of selective merging. Surface tension can bemodified through manipulation of surfactant concentrations inside theplugs. Viscosity may be also controlled, for example by addingcomponents that would renfer the desiccant plugs viscous (for example,polyethylene glycols). In other embodiments, flow rates of the array ofthe plugs and the protein stream may also be used to control ofselective merging.

A series of experiments demonstrating selective merging usingprecipitant solutions and detergent solutions were performed. In thefirst experiment, a plug containing precipitants merged with a stream of1% solution of octyl-β-D-glucopyranoside (OG) detergent. In a secondexperiment, performed on a separate but identical device to that used inthe first experiment, and at the same flow rates, a desiccant plug,containing polyethylene glycol 5000 monomethylether and a mixture ofsemi-fluorinated and non-fluorinated surfactants, bent around a streamof 1% OG, and did not merge. In a third experiment, in which a stream of1% OG was used in a combining component along with an array ofprecipitant plugs alternating with desiccant plugs, the stream of 1% OGmerged with only the precipitant plugs.

Therefore, the salt concentration in the plug that is not merged can behigher than that in the plug that is merged. If the oil is waterpermeable, the osmotic pressure resulting from the concentrationdifferent can force water to diffuse from the merged plugs to theunmerged plugs. The merged plugs can be concentrated in this way andprotein nucleation and crystallization can be enhanced.

Example 7 Crystallization of a Membrane Protein

Fatty acid amide hydrolase (FAAH) is an integral membrane protein thatdegrades endocannabinoid signaling molecules at neuron surfaces. FAAHremoval is associated with increased analgesic effects; therefore, it isconsidered a prime drug target for the relief of pain and relatedneurological disorders. The structure of rat FAAH complexed with anarachidonyl inhibitor is known. The complex showed how FAAH isintegrated into cell membranes and how it establishes access to thebilayer from its active site.

FAAH crystals were generated on chip using the microbatch method ofprotein crystallization. A composite PDMS-teflon capillary device withthree aqueous inlets and one carrier fluid inlet was constructed. Thisdevice had the capillary inserted from the outlet directly up to thecarrier fluid-aqueous junction, to facilitate the formation of plugscontaining membrane protein surfactants. The carrier fluid used wasperfluorotripentylamine (3M FC-70), which we have shown to be compatiblewith membrane protein surfactants. Using this device, ˜40 cm of tubingwas filled with the microbatch droplets. A gradient of precipitantconcentrations in the plugs was achieved by varying the flow rates ofthe precipitant and buffer streams. The precipitant solution for FAAHcomplexed with the arachidonyl inhibitor was composed of 16% PEG 6000,200 mM Li₂SO₄, 10% ethylene glycol, 14% 2-methyl-2,4-pentanediol and 200mM citrate buffered at a pH of 5.6. The precipitant for FAAH in its apoform was 24% PEG 4000, 200 mM Li₂SO₄, 10% 2-methyl-2,4-pentanediol, 400mM NaCl and 100 mM Tris buffered at a pH of 8.2.

Example 8 VD Crystallization Screen: Preparation and Transportation ofan Array of Plugs

First a small piece of Teflon tubing (outer diameter 250 μm, innerdiameter 200 μm) was connected to a 10 μL syringe (Hamilton). Thisconnection was made by using a “tube within tube” method. A thickerpiece of Teflon tubing (approximately 30 G) was connected to thesyringe, and the thing piece of tubing was inserted into the thickerpiece, and the connection was sealed (for example with was). The piecesof tubing could be stretched to reduce their diameter. The syringe andthe Teflon tubing were filled with fluorocarbon carrier fluid. Thefluorocarbon is a mixture of perfluoroperhydrophenanthrene (PFP) and1H,1H,2H,2H-perfluorooctanol (PFO) at volume ratio of 10:1. To preparean array of plugs, different reagents, fluorocarbon, and air bubbleswere aspirated successively into the piece of small Teflon tubing. Thisis achieved by pulling the plunger of the syringe that was connectedwith the Teflon tubing while the other end of the Teflon tubing was inthe corresponding solution. During the aspiration process, the movementof the plunger was controlled by a manual micrometer syringe driver(Stoelting Co.). Before the aspiration of each reagent, a small amountof fluorocarbon was aspirated into the Teflon tubing to separate theplugs. In most experiments, an air bubble was also aspirated into theTeflon tubing after the fluorocarbon aspiration. These bubbles preventedaccidental coalescence of plugs containing solutions of contrastingviscosities. After the array of the plugs was prepared inside the Teflontubing, more fluorocarbon was aspirated into the tubing. The Teflontubing was then inserted into a funnel-shaped capillary (HamptonResearch). By pushing the plunger of the syringe that was connected tothe Teflon tubing, the array of the plugs was transferred into thecapillary (OD 0.20 mm, ID 0.18 mm). The capillary was then cut off fromthe tubing and sealed by wax for long-term storage. To utilize the arrayfor experiments in microfluidic channels, it was first transported fromthe capillary into the Teflon tubing. The Teflon tubing was theninserted into a funnel-shaped adapter that was coupled to the inlet ofthe microfluidic channel. The transportation of the array was controlledby the syringe that was connected to the Teflon tubing. At the sametime, a stream of the target solution was injected into the side channeland this stream merged with the array of the plugs, mixed, and thereaction occurred. The array of the plugs after mixing was transportedinto a capillary using a similar funnel-shaped adapter. After thecapillary was filled with the array of the plugs, the capillary could betaken off the microfluidic channel and sealed for incubation oranalysis.

Example 9 Functional Assay of Several Enzymes Against One Substrate

An array of plugs was prepared, which contained alkaline phosphatase(AP) (0.02 mg/ml in 0.2 M diethanol amine, pH 10.5), catalase (0.02mg/ml in PBS, pH 7.3), ribonuclease A (RNase) (0.02 mg/ml in 0.05 M Trisbuffer, pH 7.5), and lysozyme (0.02 mg/ml in 0.05 M NaAc buffer, pH4.5). The array had one plug of each enzyme, and every two neighboringenzyme plugs were separated by two plugs of PBS buffer. These plugsserved as “washer plugs”, removing any cross-contamination of thesubstrate stream (this cross-contamination could occur during mergingwith the stream of substrate if an enzyme was transported by diffusionor surface tension-driven flow into the stream of the substrate). An airbubble was inserted between every two neighboring aqueous plugs. Toassay the activity of the four enzymes on the substrate fluoresceindiphosphate (FDP) (11 μM with 0.5 M NaCl), the array of plugs was mergedwith the solution of FDP at a T-junction (See FIG. 11). The flow ratesof the array and the FDP stream were 1.2 μL/min and 0.5 μL/min,respectively. The array of the merged plugs was collected in a capillaryand the fluorescence image of each plug was taken by a fluorescencemicroscope (Leica DMIRE2) equipped with a digital camera (Hamamatsu,ORCA-ER). Fluorescence intensity in the images was analyzed usingMetamorph Imaging System (Universal Imaging), indicating the activity ofalkaline phosphatase.

Example 10 VD Crystallization Screen: Screening Precipitants forCrystallization Conditions of Thaumatin

To screen the 48 precipitants from the Crystal Screen kit (HamptonResearch), an array of 48 plugs of 48 precipitants (from No. 1 to No.48) was prepared. The reagent formulation of the precipitants can befound at the website of Hampton Research(http://www.hamptonresearch.com/support/guides/2110F.pdf). Theprecipitant reagents had various salt concentrations (e.g., No. 44 0.2 Mammonium formate and No. 33 4.0 M sodium formate) and variousviscosities (e.g., No. 30, 30% w/v PEG 8000). Every two neighboringplugs were separated by an air bubble. The carrier fluid was the mixtureof PFP and PFO (volume ratio 10:1). After the array was prepared, it wastransported into the microchannel for screening. Another piece of Teflontubing was connected to a 10 μL syringe and the syringe+tubing assemblywas filled with PFP. Slightly less than 1.0 μL solution of thaumatin (60mg/mL in 0.1 M N-(2-acetamido)iminodiacetic acid buffer, pH 6.5) wasaspirated into the tubing. The tubing was then inserted into the inletof the side microchannel of the T-junction. The solution of thaumatinwas driven into the main channel and merged with the array of the plugsof precipitants. The flow rates of the thaumatin solution and the arraywere 0.5 μL/min and 1.2 μL/min, respectively. After the plugs weremerged with thaumatin solution, they were collected in a capillary andsealed by wax for incubation. Incubation of the plugs at 18° C. resultedin crystallization of thaumatin in plugs that contained precipitant No.29 and thaumatin. Less than ˜0.1 μL of thaumatin solution remained inthe channel. This method does not require large volumes of solution anddoes not generate much waste, and is useful with volumes as low assub-microliter, screened against nanoliter plugs of multiple reagents.

To screen the five precipitants from the Crystal Screen kit, an array ofplugs was prepared, which contained five different precipitants from thescreening kit: No. 13 (0.2 M sodium citrate/0.1 M tris HCl/30% v/v PEG400, pH 8.5), No. 24 (0.2 M CaCl2/0.1 M NaAc/20% v/v isopropanol, pH4.6), No. 25 (0.1 M imidazole/1.0 M NaAc, pH 6.5), No. 29 (0.1 MHEPES/0.8 M potassium sodium tartrate, pH 7.5), and No. 33 (4.0 M sodiumformate). The array had two plugs of each precipitant and totally tenplugs. An air bubble was inserted between every two neighboring plugs.The mixture of PFP and PFO (volume ratio 10:1) was used as the carrierfluid. An aqueous stream of thaumatin (˜60 mg/ml in 0.1 M ADA buffer, pH6.5) was injected into the array of the plugs for crystallization. Theflow rates of the array and the stream of thaumatin were 1.2 μL/min and0.5 μL/min, respectively. Incubation of the plugs at 18° C. resulted incrystallization of thaumatin in plugs that contained precipitant No. 29and thaumatin.

Example 11 Free-Interface Diffusion Crystallization Screen

Free interface diffusion trials were set up by alternating protein plugswith precipitant plugs. The alternating pairs of plugs were pumped intoa piece of narrow Teflon tubing, prior to being transferred to a sealedcapillary. (The sealed capillary was rendered hydrophobic bysilanization). A hydrophilic glass fiber was inserted into thecapillary. The capillary was moved at a constant rate to control thedistance between the plugs as they were deposited. The dropletsspontaneously wetted the glass fiber forming a small interconnectionbetween the drops allowing diffusion to occur.

Example 12 Free-Interface Diffusion Crystallization Screen

A Teflon capillary (ID 0.008±0.001 in.; wall 0.001±0.001 in.) was filledwith a 5:1 v/v mixture of 3M FC-3283:1H,1H,2H,2H-perfluoro-1-octanol. A˜300 nL plug of 25 mg/mL thaumatin in ADA buffer was aspirated into thecapillary followed by a small ˜200 nL slug of the fluorocarbon mixture,a ˜300 nL plug of 2 M sodium/potassium tartrate, and another slug of thefluorocarbon mixture, in that order. The open ends of the capillary weresealed with capillary wax, and the capillary was secured in an eppendorf5415 D centrifuge such that the accelerating force was exertedapproximately normal to the length of the capillary. (The capillary wasplaced on the rotor and secured with tape to the top of centrifugetubes.) Centrifugation was applied at 2000 rpm for 30 s. Thecentrifugation process dispelled the denser carrier fluid from betweenthe two aqueous plugs, allowing the two aqueous plugs to come togetherand establish an interface. Solutes mixed diffusively across theinterface, and a precipitate was observed at the interface between thetwo aqueous plugs immediately after centrifugation. One day later,crystals of the protein thaumatin formed.

The invention claimed is:
 1. A microfluidic system, the system comprising: a microfluidic loading component comprising a microchannel and a T-junction, wherein the microchannel and the T-junction are in fluid communication and the T-junction comprises an inlet and two outlets each in fluid communication with a plurality of downstream T-junctions, wherein each of the downstream T-junctions comprises two outlets and wherein the downstream T-junctions are parallel to each other; a plurality of plugs in an immiscible carrier fluid, each plug comprising a plug fluid substantially surrounded by the immiscible carrier fluid, wherein the plug fluids of at least some of the plurality of plugs comprise a biological molecule; and a plurality of detachable holding components, wherein each detachable holding component comprises an inlet detachably couplable to and in fluid communication with an outlet of one of the plurality of downstream T-junctions, wherein the microfluidic system is configured such that the plurality of plugs are flowing through the loading component, the loading component splits the plurality of plugs at each of the plurality of downstream T-junctions and each inlet of the plurality of detachable holding components is configured to be operably coupled with at least one of the outlets of the plurality of downstream T-junctions in a manner that at least one holding component receives a plurality of split plugs in the immiscible carrier fluid from the loading component.
 2. The microfluidic system according to claim 1, wherein each plug comprises at least one solvent and at least one reagent.
 3. The microfluidic system according to claim 1, wherein the carrier fluid is a fluorinated carrier fluid.
 4. The microfluidic system according to claim 1, wherein the carrier fluid is an oil.
 5. The microfluidic system according to claim 4, wherein the oil comprises a surfactant.
 6. The microfluidic system according to claim 5, wherein the surfactant is a fluorinated surfactant.
 7. The microfluidic system according to claim 5, wherein the surfactant comprises oligoethylene glycol groups.
 8. The microfluidic system according to claim 1, further comprising a receiving component, the receiving component comprising: a microchannel comprising an inlet configured to fluidically couple to at least one of the plurality of holding components in order to receive a plurality of plugs from the plurality of holding component.
 9. The microfluidic system according to claim 1, wherein at least one of the plurality of holding components is different than another one of the plurality of holding components.
 10. The microfluidic system according to claim 1, wherein all of the plurality of holding components are the same.
 11. The microfluidic system according to claim 1, wherein one or more of the T-junction and downstream T-junctions comprises a third outlet.
 12. The microfluidic system according to claim 1, wherein one or more of the T-junction and downstream T-junctions is a narrowed T-junction. 